diff --git a/folly/FixedString.h b/folly/FixedString.h
index ec7c668b2c1..40ba56ee84a 100644
--- a/folly/FixedString.h
+++ b/folly/FixedString.h
@@ -307,7 +307,7 @@ struct Helper {
 #endif
 
 template <class T>
-FOLLY_CPP14_CONSTEXPR void constexpr_swap(T& a, T& b) noexcept(
+constexpr void constexpr_swap(T& a, T& b) noexcept(
     noexcept(a = T(std::move(a)))) {
   T tmp((std::move(a)));
   a = std::move(b);
@@ -336,7 +336,7 @@ struct ReverseIterator {
 
   constexpr ReverseIterator() = default;
   constexpr ReverseIterator(const ReverseIterator&) = default;
-  FOLLY_CPP14_CONSTEXPR ReverseIterator& operator=(const ReverseIterator&) =
+  constexpr ReverseIterator& operator=(const ReverseIterator&) =
       default;
   constexpr explicit ReverseIterator(T* p) noexcept : p_(p) {}
   constexpr /* implicit */ ReverseIterator(const other& that) noexcept
@@ -354,25 +354,25 @@ struct ReverseIterator {
   constexpr reference operator*() const {
     return *(p_ - 1);
   }
-  FOLLY_CPP14_CONSTEXPR ReverseIterator& operator++() noexcept {
+  constexpr ReverseIterator& operator++() noexcept {
     --p_;
     return *this;
   }
-  FOLLY_CPP14_CONSTEXPR ReverseIterator operator++(int) noexcept {
+  constexpr ReverseIterator operator++(int) noexcept {
     auto tmp(*this);
     --p_;
     return tmp;
   }
-  FOLLY_CPP14_CONSTEXPR ReverseIterator& operator--() noexcept {
+  constexpr ReverseIterator& operator--() noexcept {
     ++p_;
     return *this;
   }
-  FOLLY_CPP14_CONSTEXPR ReverseIterator operator--(int) noexcept {
+  constexpr ReverseIterator operator--(int) noexcept {
     auto tmp(*this);
     ++p_;
     return tmp;
   }
-  FOLLY_CPP14_CONSTEXPR ReverseIterator& operator+=(std::ptrdiff_t i) noexcept {
+  constexpr ReverseIterator& operator+=(std::ptrdiff_t i) noexcept {
     p_ -= i;
     return *this;
   }
@@ -386,7 +386,7 @@ struct ReverseIterator {
       std::ptrdiff_t i) noexcept {
     return ReverseIterator{that.p_ - i};
   }
-  FOLLY_CPP14_CONSTEXPR ReverseIterator& operator-=(std::ptrdiff_t i) noexcept {
+  constexpr ReverseIterator& operator-=(std::ptrdiff_t i) noexcept {
     p_ += i;
     return *this;
   }
@@ -728,7 +728,7 @@ class BasicFixedString : private detail::fixedstring::FixedStringBase {
   constexpr BasicFixedString(std::initializer_list<Char> il) noexcept(false)
       : BasicFixedString{il.begin(), il.size()} {}
 
-  FOLLY_CPP14_CONSTEXPR BasicFixedString& operator=(
+  constexpr BasicFixedString& operator=(
       const BasicFixedString&) noexcept = default;
 
   /** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **
@@ -745,7 +745,7 @@ class BasicFixedString : private detail::fixedstring::FixedStringBase {
    * \return `*this`
    */
   template <std::size_t M>
-  FOLLY_CPP14_CONSTEXPR BasicFixedString& operator=(
+  constexpr BasicFixedString& operator=(
       const BasicFixedString<Char, M>& that) noexcept(M <= N) {
     detail::fixedstring::checkOverflow(that.size_, N);
     size_ = that.copy(data_, that.size_);
@@ -764,7 +764,7 @@ class BasicFixedString : private detail::fixedstring::FixedStringBase {
    * \return `*this`
    */
   template <std::size_t M, class = typename std::enable_if<(M - 1u <= N)>::type>
-  FOLLY_CPP14_CONSTEXPR BasicFixedString& operator=(
+  constexpr BasicFixedString& operator=(
       const Char (&that)[M]) noexcept {
     return assign(detail::fixedstring::checkNullTerminated(that), M - 1u);
   }
@@ -778,7 +778,7 @@ class BasicFixedString : private detail::fixedstring::FixedStringBase {
    * \throw std::out_of_range when il.size() > N
    * \return `*this`
    */
-  FOLLY_CPP14_CONSTEXPR BasicFixedString& operator=(
+  constexpr BasicFixedString& operator=(
       std::initializer_list<Char> il) noexcept(false) {
     detail::fixedstring::checkOverflow(il.size(), N);
     for (std::size_t i = 0u; i < il.size(); ++i) {
@@ -793,7 +793,7 @@ class BasicFixedString : private detail::fixedstring::FixedStringBase {
    * Conversion to folly::Range
    * \return `Range<Char*>{begin(), end()}`
    */
-  FOLLY_CPP14_CONSTEXPR Range<Char*> toRange() noexcept {
+  constexpr Range<Char*> toRange() noexcept {
     return {begin(), end()};
   }
 
@@ -821,7 +821,7 @@ class BasicFixedString : private detail::fixedstring::FixedStringBase {
   // to compile. But it creates ambiguities when passing a FixedString to an
   // API that has overloads for `const char*` and `folly::Range`, for instance.
   // using ArrayType = Char[N];
-  // FOLLY_CPP14_CONSTEXPR /* implicit */ operator ArrayType&() noexcept {
+  // constexpr /* implicit */ operator ArrayType&() noexcept {
   //   return data_;
   // }
 
@@ -841,7 +841,7 @@ class BasicFixedString : private detail::fixedstring::FixedStringBase {
    * \throw std::out_of_range when count > N
    * \return `*this`
    */
-  FOLLY_CPP14_CONSTEXPR BasicFixedString& assign(
+  constexpr BasicFixedString& assign(
       std::size_t count,
       Char ch) noexcept(false) {
     detail::fixedstring::checkOverflow(count, N);
@@ -858,7 +858,7 @@ class BasicFixedString : private detail::fixedstring::FixedStringBase {
    * \note Equivalent to `assign(that, 0, that.size())`
    */
   template <std::size_t M>
-  FOLLY_CPP14_CONSTEXPR BasicFixedString& assign(
+  constexpr BasicFixedString& assign(
       const BasicFixedString<Char, M>& that) noexcept(M <= N) {
     return *this = that;
   }
@@ -868,7 +868,7 @@ class BasicFixedString : private detail::fixedstring::FixedStringBase {
   // N is a count of characters. In the latter, it would be a position, which
   // totally changes the meaning of the code.
   template <std::size_t M>
-  FOLLY_CPP14_CONSTEXPR BasicFixedString& assign(
+  constexpr BasicFixedString& assign(
       const BasicFixedString<Char, M>& that,
       std::size_t pos) noexcept(false) = delete;
 
@@ -890,7 +890,7 @@ class BasicFixedString : private detail::fixedstring::FixedStringBase {
    * \return `*this`
    */
   template <std::size_t M>
-  FOLLY_CPP14_CONSTEXPR BasicFixedString& assign(
+  constexpr BasicFixedString& assign(
       const BasicFixedString<Char, M>& that,
       std::size_t pos,
       std::size_t count) noexcept(false) {
@@ -907,7 +907,7 @@ class BasicFixedString : private detail::fixedstring::FixedStringBase {
    * \note Equivalent to `assign(that, M - 1)`
    */
   template <std::size_t M, class = typename std::enable_if<(M - 1u <= N)>::type>
-  FOLLY_CPP14_CONSTEXPR BasicFixedString& assign(
+  constexpr BasicFixedString& assign(
       const Char (&that)[M]) noexcept {
     return assign(detail::fixedstring::checkNullTerminated(that), M - 1u);
   }
@@ -924,7 +924,7 @@ class BasicFixedString : private detail::fixedstring::FixedStringBase {
    * \throw std::out_of_range when count > N
    * \return `*this`
    */
-  FOLLY_CPP14_CONSTEXPR BasicFixedString& assign(
+  constexpr BasicFixedString& assign(
       const Char* that,
       std::size_t count) noexcept(false) {
     detail::fixedstring::checkOverflow(count, N);
@@ -939,7 +939,7 @@ class BasicFixedString : private detail::fixedstring::FixedStringBase {
   /** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **
    * Swap the contents of this string with `that`.
    */
-  FOLLY_CPP14_CONSTEXPR void swap(BasicFixedString& that) noexcept {
+  constexpr void swap(BasicFixedString& that) noexcept {
     // less-than-or-equal here to copy the null terminator:
     for (std::size_t i = 0u; i <= folly::constexpr_max(size_, that.size_);
          ++i) {
@@ -952,7 +952,7 @@ class BasicFixedString : private detail::fixedstring::FixedStringBase {
    * Return a pointer to a range of `size()+1` characters, the last of which
    * is `Char(0)`.
    */
-  FOLLY_CPP14_CONSTEXPR Char* data() noexcept {
+  constexpr Char* data() noexcept {
     return data_;
   }
 
@@ -973,7 +973,7 @@ class BasicFixedString : private detail::fixedstring::FixedStringBase {
   /** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **
    * \return `data()`.
    */
-  FOLLY_CPP14_CONSTEXPR Char* begin() noexcept {
+  constexpr Char* begin() noexcept {
     return data_;
   }
 
@@ -994,7 +994,7 @@ class BasicFixedString : private detail::fixedstring::FixedStringBase {
   /** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **
    * \return `data() + size()`.
    */
-  FOLLY_CPP14_CONSTEXPR Char* end() noexcept {
+  constexpr Char* end() noexcept {
     return data_ + size_;
   }
 
@@ -1016,7 +1016,7 @@ class BasicFixedString : private detail::fixedstring::FixedStringBase {
    * Returns a reverse iterator to the first character of the reversed string.
    * It corresponds to the last + 1 character of the non-reversed string.
    */
-  FOLLY_CPP14_CONSTEXPR reverse_iterator rbegin() noexcept {
+  constexpr reverse_iterator rbegin() noexcept {
     return reverse_iterator{data_ + size_};
   }
 
@@ -1038,7 +1038,7 @@ class BasicFixedString : private detail::fixedstring::FixedStringBase {
    * Returns a reverse iterator to the last + 1 character of the reversed
    * string. It corresponds to the first character of the non-reversed string.
    */
-  FOLLY_CPP14_CONSTEXPR reverse_iterator rend() noexcept {
+  constexpr reverse_iterator rend() noexcept {
     return reverse_iterator{data_};
   }
 
@@ -1102,7 +1102,7 @@ class BasicFixedString : private detail::fixedstring::FixedStringBase {
    * \return `*(data() + i)`
    * \throw std::out_of_range when i > size()
    */
-  FOLLY_CPP14_CONSTEXPR Char& at(std::size_t i) noexcept(false) {
+  constexpr Char& at(std::size_t i) noexcept(false) {
     return i <= size_ ? data_[i]
                       : (throw_exception<std::out_of_range>(
                              "Out of range in BasicFixedString::at"),
@@ -1124,7 +1124,7 @@ class BasicFixedString : private detail::fixedstring::FixedStringBase {
    * \note `(*this)[size()]` is allowed will return `Char(0)`.
    * \return `*(data() + i)`
    */
-  FOLLY_CPP14_CONSTEXPR Char& operator[](std::size_t i) noexcept {
+  constexpr Char& operator[](std::size_t i) noexcept {
 #ifdef NDEBUG
     return data_[i];
 #else
@@ -1146,7 +1146,7 @@ class BasicFixedString : private detail::fixedstring::FixedStringBase {
   /**
    * \note Equivalent to `(*this)[0]`
    */
-  FOLLY_CPP14_CONSTEXPR Char& front() noexcept {
+  constexpr Char& front() noexcept {
     return (*this)[0u];
   }
 
@@ -1161,7 +1161,7 @@ class BasicFixedString : private detail::fixedstring::FixedStringBase {
    * \note Equivalent to `at(size()-1)`
    * \pre `!empty()`
    */
-  FOLLY_CPP14_CONSTEXPR Char& back() noexcept {
+  constexpr Char& back() noexcept {
 #ifdef NDEBUG
     return data_[size_ - 1u];
 #else
@@ -1185,7 +1185,7 @@ class BasicFixedString : private detail::fixedstring::FixedStringBase {
    * \post `size() == 0u`
    * \post `at(size()) == Char(0)`
    */
-  FOLLY_CPP14_CONSTEXPR void clear() noexcept {
+  constexpr void clear() noexcept {
     data_[0u] = Char(0);
     size_ = 0u;
   }
@@ -1193,7 +1193,7 @@ class BasicFixedString : private detail::fixedstring::FixedStringBase {
   /**
    * \note Equivalent to `append(1u, ch)`.
    */
-  FOLLY_CPP14_CONSTEXPR void push_back(Char ch) noexcept(false) {
+  constexpr void push_back(Char ch) noexcept(false) {
     detail::fixedstring::checkOverflow(1u, N - size_);
     data_[size_] = ch;
     data_[++size_] = Char(0);
@@ -1214,7 +1214,7 @@ class BasicFixedString : private detail::fixedstring::FixedStringBase {
    * \post The characters in the half-open range `[0,size()-1)` are unmodified.
    * \throw std::out_of_range if empty().
    */
-  FOLLY_CPP14_CONSTEXPR void pop_back() noexcept(false) {
+  constexpr void pop_back() noexcept(false) {
     detail::fixedstring::checkOverflow(1u, size_);
     --size_;
     data_[size_] = Char(0);
@@ -1237,7 +1237,7 @@ class BasicFixedString : private detail::fixedstring::FixedStringBase {
    * \post `size() == old_size + count`
    * \throw std::out_of_range if count > N - size().
    */
-  FOLLY_CPP14_CONSTEXPR BasicFixedString& append(
+  constexpr BasicFixedString& append(
       std::size_t count,
       Char ch) noexcept(false) {
     detail::fixedstring::checkOverflow(count, N - size_);
@@ -1253,7 +1253,7 @@ class BasicFixedString : private detail::fixedstring::FixedStringBase {
    * \note Equivalent to `append(*this, 0, that.size())`.
    */
   template <std::size_t M>
-  FOLLY_CPP14_CONSTEXPR BasicFixedString& append(
+  constexpr BasicFixedString& append(
       const BasicFixedString<Char, M>& that) noexcept(false) {
     return append(that, 0u, that.size_);
   }
@@ -1262,7 +1262,7 @@ class BasicFixedString : private detail::fixedstring::FixedStringBase {
   // append("null-terminated", N), where N would be a count instead
   // of a position.
   template <std::size_t M>
-  FOLLY_CPP14_CONSTEXPR BasicFixedString& append(
+  constexpr BasicFixedString& append(
       const BasicFixedString<Char, M>& that,
       std::size_t pos) noexcept(false) = delete;
 
@@ -1283,7 +1283,7 @@ class BasicFixedString : private detail::fixedstring::FixedStringBase {
    *        `old_size + count > N`.
    */
   template <std::size_t M>
-  FOLLY_CPP14_CONSTEXPR BasicFixedString& append(
+  constexpr BasicFixedString& append(
       const BasicFixedString<Char, M>& that,
       std::size_t pos,
       std::size_t count) noexcept(false) {
@@ -1301,7 +1301,7 @@ class BasicFixedString : private detail::fixedstring::FixedStringBase {
   /**
    * \note Equivalent to `append(that, strlen(that))`.
    */
-  FOLLY_CPP14_CONSTEXPR BasicFixedString& append(const Char* that) noexcept(
+  constexpr BasicFixedString& append(const Char* that) noexcept(
       false) {
     return append(that, folly::constexpr_strlen(that));
   }
@@ -1315,7 +1315,7 @@ class BasicFixedString : private detail::fixedstring::FixedStringBase {
    * \post `at(size()) == Char(0)`
    * \throw std::out_of_range if old_size + count > N.
    */
-  FOLLY_CPP14_CONSTEXPR BasicFixedString& append(
+  constexpr BasicFixedString& append(
       const Char* that,
       std::size_t count) noexcept(false) {
     detail::fixedstring::checkOverflow(count, N - size_);
@@ -1401,7 +1401,7 @@ class BasicFixedString : private detail::fixedstring::FixedStringBase {
    * Appends characters from a null-terminated string literal to this string.
    * \note Equivalent to `append(that)`.
    */
-  FOLLY_CPP14_CONSTEXPR BasicFixedString& operator+=(const Char* that) noexcept(
+  constexpr BasicFixedString& operator+=(const Char* that) noexcept(
       false) {
     return append(that);
   }
@@ -1411,7 +1411,7 @@ class BasicFixedString : private detail::fixedstring::FixedStringBase {
    * \note Equivalent to `append(that)`.
    */
   template <std::size_t M>
-  FOLLY_CPP14_CONSTEXPR BasicFixedString& operator+=(
+  constexpr BasicFixedString& operator+=(
       const BasicFixedString<Char, M>& that) noexcept(false) {
     return append(that, 0u, that.size_);
   }
@@ -1420,7 +1420,7 @@ class BasicFixedString : private detail::fixedstring::FixedStringBase {
    * Appends a character to this string.
    * \note Equivalent to `push_back(ch)`.
    */
-  FOLLY_CPP14_CONSTEXPR BasicFixedString& operator+=(Char ch) noexcept(false) {
+  constexpr BasicFixedString& operator+=(Char ch) noexcept(false) {
     push_back(ch);
     return *this;
   }
@@ -1429,7 +1429,7 @@ class BasicFixedString : private detail::fixedstring::FixedStringBase {
    * Appends characters from an `initializer_list` to this string.
    * \note Equivalent to `append(il.begin(), il.size())`.
    */
-  FOLLY_CPP14_CONSTEXPR BasicFixedString& operator+=(
+  constexpr BasicFixedString& operator+=(
       std::initializer_list<Char> il) noexcept(false) {
     return append(il.begin(), il.size());
   }
@@ -1439,7 +1439,7 @@ class BasicFixedString : private detail::fixedstring::FixedStringBase {
    * \note Equivalent to `clear()`
    * \return *this;
    */
-  FOLLY_CPP14_CONSTEXPR BasicFixedString& erase() noexcept {
+  constexpr BasicFixedString& erase() noexcept {
     clear();
     return *this;
   }
@@ -1454,7 +1454,7 @@ class BasicFixedString : private detail::fixedstring::FixedStringBase {
    * \return *this;
    * \throw std::out_of_range when pos > size().
    */
-  FOLLY_CPP14_CONSTEXPR BasicFixedString& erase(
+  constexpr BasicFixedString& erase(
       std::size_t pos,
       std::size_t count = npos) noexcept(false) {
     using A = const Char[1];
@@ -1471,7 +1471,7 @@ class BasicFixedString : private detail::fixedstring::FixedStringBase {
    * \note Equivalent to `erase(first - data(), 1)`
    * \return A pointer to the first character after the erased character.
    */
-  FOLLY_CPP14_CONSTEXPR Char* erase(const Char* first) noexcept(false) {
+  constexpr Char* erase(const Char* first) noexcept(false) {
     erase(first - data_, 1u);
     return data_ + (first - data_);
   }
@@ -1480,7 +1480,7 @@ class BasicFixedString : private detail::fixedstring::FixedStringBase {
    * \note Equivalent to `erase(first - data(), last - first)`
    * \return A pointer to the first character after the erased characters.
    */
-  FOLLY_CPP14_CONSTEXPR Char* erase(
+  constexpr Char* erase(
       const Char* first,
       const Char* last) noexcept(false) {
     erase(first - data_, last - first);
@@ -1661,7 +1661,7 @@ class BasicFixedString : private detail::fixedstring::FixedStringBase {
    *   `replace(first - data(), last - first, that.data(), that.size())`
    */
   template <std::size_t M>
-  FOLLY_CPP14_CONSTEXPR BasicFixedString& replace(
+  constexpr BasicFixedString& replace(
       const Char* first,
       const Char* last,
       const BasicFixedString<Char, M>& that) noexcept(false) {
@@ -1677,7 +1677,7 @@ class BasicFixedString : private detail::fixedstring::FixedStringBase {
    *   that.size() - that_pos)</tt>
    */
   template <std::size_t M>
-  FOLLY_CPP14_CONSTEXPR BasicFixedString& replace(
+  constexpr BasicFixedString& replace(
       std::size_t this_pos,
       std::size_t this_count,
       const BasicFixedString<Char, M>& that,
@@ -1694,7 +1694,7 @@ class BasicFixedString : private detail::fixedstring::FixedStringBase {
    *   `replace(this_pos, this_count, that.data() + that_pos, that_count)`
    */
   template <std::size_t M>
-  FOLLY_CPP14_CONSTEXPR BasicFixedString& replace(
+  constexpr BasicFixedString& replace(
       std::size_t this_pos,
       std::size_t this_count,
       const BasicFixedString<Char, M>& that,
@@ -1709,7 +1709,7 @@ class BasicFixedString : private detail::fixedstring::FixedStringBase {
    * \note Equivalent to
    *   `replace(this_pos, this_count, that, strlen(that))`
    */
-  FOLLY_CPP14_CONSTEXPR BasicFixedString& replace(
+  constexpr BasicFixedString& replace(
       std::size_t this_pos,
       std::size_t this_count,
       const Char* that) noexcept(false) {
@@ -1724,7 +1724,7 @@ class BasicFixedString : private detail::fixedstring::FixedStringBase {
    * \note Equivalent to
    *   `replace(first - data(), last - first, that, strlen(that))`
    */
-  FOLLY_CPP14_CONSTEXPR BasicFixedString& replace(
+  constexpr BasicFixedString& replace(
       const Char* first,
       const Char* last,
       const Char* that) noexcept(false) {
@@ -1749,7 +1749,7 @@ class BasicFixedString : private detail::fixedstring::FixedStringBase {
    *   - `this_count > size() - this_pos`
    *   - `size() - this_count + that_count > N`
    */
-  FOLLY_CPP14_CONSTEXPR BasicFixedString& replace(
+  constexpr BasicFixedString& replace(
       std::size_t this_pos,
       std::size_t this_count,
       const Char* that,
@@ -1772,7 +1772,7 @@ class BasicFixedString : private detail::fixedstring::FixedStringBase {
    * \note Equivalent to
    *   `replace(this_pos, this_count, BasicFixedString{that_count, ch})`
    */
-  FOLLY_CPP14_CONSTEXPR BasicFixedString& replace(
+  constexpr BasicFixedString& replace(
       std::size_t this_pos,
       std::size_t this_count,
       std::size_t that_count,
@@ -1786,7 +1786,7 @@ class BasicFixedString : private detail::fixedstring::FixedStringBase {
    * \note Equivalent to
    *   `replace(first - data(), last - first, BasicFixedString{that_count, ch})`
    */
-  FOLLY_CPP14_CONSTEXPR BasicFixedString& replace(
+  constexpr BasicFixedString& replace(
       const Char* first,
       const Char* last,
       std::size_t that_count,
@@ -1803,7 +1803,7 @@ class BasicFixedString : private detail::fixedstring::FixedStringBase {
    * \note Equivalent to
    *   `replace(this_pos, this_count, il.begin(), il.size())`
    */
-  FOLLY_CPP14_CONSTEXPR BasicFixedString& replace(
+  constexpr BasicFixedString& replace(
       const Char* first,
       const Char* last,
       std::initializer_list<Char> il) noexcept(false) {
@@ -2014,7 +2014,7 @@ class BasicFixedString : private detail::fixedstring::FixedStringBase {
    *   from this string into the buffer pointed to by `dest`.
    * \return The number of characters copied.
    */
-  FOLLY_CPP14_CONSTEXPR std::size_t copy(Char* dest, std::size_t count) const
+  constexpr std::size_t copy(Char* dest, std::size_t count) const
       noexcept {
     return copy(dest, count, 0u);
   }
@@ -2026,7 +2026,7 @@ class BasicFixedString : private detail::fixedstring::FixedStringBase {
    * \return The number of characters copied.
    * \throw std::out_of_range if `pos > size()`
    */
-  FOLLY_CPP14_CONSTEXPR std::size_t
+  constexpr std::size_t
   copy(Char* dest, std::size_t count, std::size_t pos) const noexcept(false) {
     detail::fixedstring::checkOverflow(pos, size_);
     for (std::size_t i = 0u; i < count; ++i) {
@@ -2042,7 +2042,7 @@ class BasicFixedString : private detail::fixedstring::FixedStringBase {
    * Resizes the current string.
    * \note Equivalent to `resize(count, Char(0))`
    */
-  FOLLY_CPP14_CONSTEXPR void resize(std::size_t count) noexcept(false) {
+  constexpr void resize(std::size_t count) noexcept(false) {
     resize(count, Char(0));
   }
 
@@ -2051,7 +2051,7 @@ class BasicFixedString : private detail::fixedstring::FixedStringBase {
    *   `data()[count]` to `Char(0)`. If `count > old_size`, the characters
    *   in the range [`old_size`,`count`) are set to `ch`.
    */
-  FOLLY_CPP14_CONSTEXPR void resize(std::size_t count, Char ch) noexcept(
+  constexpr void resize(std::size_t count, Char ch) noexcept(
       false) {
     detail::fixedstring::checkOverflow(count, N);
     if (count == size_) {
@@ -2998,7 +2998,7 @@ constexpr BasicFixedString<Char, N - 1u> makeFixedString(
  * Swap function
  */
 template <class Char, std::size_t N>
-FOLLY_CPP14_CONSTEXPR void swap(
+constexpr void swap(
     BasicFixedString<Char, N>& a,
     BasicFixedString<Char, N>& b) noexcept {
   a.swap(b);
diff --git a/folly/Optional.h b/folly/Optional.h
index e0c0a787c3c..53df78cc7f8 100644
--- a/folly/Optional.h
+++ b/folly/Optional.h
@@ -104,7 +104,7 @@ class Optional {
       !std::is_abstract<Value>::value,
       "Optional may not be used with abstract types");
 
-  FOLLY_CPP14_CONSTEXPR Optional() noexcept {}
+  constexpr Optional() noexcept {}
 
   Optional(const Optional& src) noexcept(
       std::is_nothrow_copy_constructible<Value>::value) {
@@ -121,14 +121,14 @@ class Optional {
     }
   }
 
-  FOLLY_CPP14_CONSTEXPR /* implicit */ Optional(const None&) noexcept {}
+  constexpr /* implicit */ Optional(const None&) noexcept {}
 
-  FOLLY_CPP14_CONSTEXPR /* implicit */ Optional(Value&& newValue) noexcept(
+  constexpr /* implicit */ Optional(Value&& newValue) noexcept(
       std::is_nothrow_move_constructible<Value>::value) {
     construct(std::move(newValue));
   }
 
-  FOLLY_CPP14_CONSTEXPR /* implicit */ Optional(const Value& newValue) noexcept(
+  constexpr /* implicit */ Optional(const Value& newValue) noexcept(
       std::is_nothrow_copy_constructible<Value>::value) {
     construct(newValue);
   }
@@ -153,12 +153,12 @@ class Optional {
            Null>::type) noexcept = delete;
 
   template <typename... Args>
-  FOLLY_CPP14_CONSTEXPR explicit Optional(in_place_t, Args&&... args) noexcept(
+  constexpr explicit Optional(in_place_t, Args&&... args) noexcept(
       std::is_nothrow_constructible<Value, Args...>::value)
       : Optional{PrivateConstructor{}, std::forward<Args>(args)...} {}
 
   template <typename U, typename... Args>
-  FOLLY_CPP14_CONSTEXPR explicit Optional(
+  constexpr explicit Optional(
       in_place_t,
       std::initializer_list<U> il,
       Args&&... args) noexcept(std::
@@ -274,22 +274,22 @@ class Optional {
     }
   }
 
-  FOLLY_CPP14_CONSTEXPR const Value& value() const& {
+  constexpr const Value& value() const& {
     require_value();
     return storage_.value;
   }
 
-  FOLLY_CPP14_CONSTEXPR Value& value() & {
+  constexpr Value& value() & {
     require_value();
     return storage_.value;
   }
 
-  FOLLY_CPP14_CONSTEXPR Value&& value() && {
+  constexpr Value&& value() && {
     require_value();
     return std::move(storage_.value);
   }
 
-  FOLLY_CPP14_CONSTEXPR const Value&& value() const&& {
+  constexpr const Value&& value() const&& {
     require_value();
     return std::move(storage_.value);
   }
@@ -302,41 +302,41 @@ class Optional {
   }
   Value* get_pointer() && = delete;
 
-  FOLLY_CPP14_CONSTEXPR bool has_value() const noexcept {
+  constexpr bool has_value() const noexcept {
     return storage_.hasValue;
   }
 
-  FOLLY_CPP14_CONSTEXPR bool hasValue() const noexcept {
+  constexpr bool hasValue() const noexcept {
     return has_value();
   }
 
-  FOLLY_CPP14_CONSTEXPR explicit operator bool() const noexcept {
+  constexpr explicit operator bool() const noexcept {
     return has_value();
   }
 
-  FOLLY_CPP14_CONSTEXPR const Value& operator*() const& {
+  constexpr const Value& operator*() const& {
     return value();
   }
-  FOLLY_CPP14_CONSTEXPR Value& operator*() & {
+  constexpr Value& operator*() & {
     return value();
   }
-  FOLLY_CPP14_CONSTEXPR const Value&& operator*() const&& {
+  constexpr const Value&& operator*() const&& {
     return std::move(value());
   }
-  FOLLY_CPP14_CONSTEXPR Value&& operator*() && {
+  constexpr Value&& operator*() && {
     return std::move(value());
   }
 
-  FOLLY_CPP14_CONSTEXPR const Value* operator->() const {
+  constexpr const Value* operator->() const {
     return &value();
   }
-  FOLLY_CPP14_CONSTEXPR Value* operator->() {
+  constexpr Value* operator->() {
     return &value();
   }
 
   // Return a copy of the value if set, or a given default if not.
   template <class U>
-  FOLLY_CPP14_CONSTEXPR Value value_or(U&& dflt) const& {
+  constexpr Value value_or(U&& dflt) const& {
     if (storage_.hasValue) {
       return storage_.value;
     }
@@ -345,7 +345,7 @@ class Optional {
   }
 
   template <class U>
-  FOLLY_CPP14_CONSTEXPR Value value_or(U&& dflt) && {
+  constexpr Value value_or(U&& dflt) && {
     if (storage_.hasValue) {
       return std::move(storage_.value);
     }
@@ -373,7 +373,7 @@ class Optional {
     explicit PrivateConstructor() = default;
   };
   template <typename... Args>
-  FOLLY_CPP14_CONSTEXPR Optional(PrivateConstructor, Args&&... args) noexcept(
+  constexpr Optional(PrivateConstructor, Args&&... args) noexcept(
       std::is_constructible<Value, Args&&...>::value) {
     construct(std::forward<Args>(args)...);
   }
@@ -493,7 +493,7 @@ constexpr bool operator!=(const U& a, const Optional<V>& b) {
 }
 
 template <class U, class V>
-FOLLY_CPP14_CONSTEXPR bool operator==(
+constexpr bool operator==(
     const Optional<U>& a,
     const Optional<V>& b) {
   if (a.hasValue() != b.hasValue()) {
@@ -511,7 +511,7 @@ constexpr bool operator!=(const Optional<U>& a, const Optional<V>& b) {
 }
 
 template <class U, class V>
-FOLLY_CPP14_CONSTEXPR bool operator<(
+constexpr bool operator<(
     const Optional<U>& a,
     const Optional<V>& b) {
   if (a.hasValue() != b.hasValue()) {
diff --git a/folly/Portability.h b/folly/Portability.h
index 4fe452a6939..a97fbbed18c 100644
--- a/folly/Portability.h
+++ b/folly/Portability.h
@@ -440,24 +440,6 @@ constexpr auto kCpplibVer = 0;
 #endif
 } // namespace folly
 
-// Define FOLLY_USE_CPP14_CONSTEXPR to be true if the compiler's C++14
-// constexpr support is "good enough".
-#ifndef FOLLY_USE_CPP14_CONSTEXPR
-#if defined(__clang__)
-#define FOLLY_USE_CPP14_CONSTEXPR __cplusplus >= 201300L
-#elif defined(__GNUC__)
-#define FOLLY_USE_CPP14_CONSTEXPR __cplusplus >= 201304L
-#else
-#define FOLLY_USE_CPP14_CONSTEXPR 0 // MSVC?
-#endif
-#endif
-
-#if FOLLY_USE_CPP14_CONSTEXPR
-#define FOLLY_CPP14_CONSTEXPR constexpr
-#else
-#define FOLLY_CPP14_CONSTEXPR inline
-#endif
-
 //  MSVC does not permit:
 //
 //    extern int const num;
@@ -471,19 +453,9 @@ constexpr auto kCpplibVer = 0;
 //  True as of MSVC 2017.
 #if _MSC_VER
 #define FOLLY_STORAGE_CONSTEXPR
-#define FOLLY_STORAGE_CPP14_CONSTEXPR
-#else
-#if __ICC
-#define FOLLY_STORAGE_CONSTEXPR
 #else
 #define FOLLY_STORAGE_CONSTEXPR constexpr
 #endif
-#if FOLLY_USE_CPP14_CONSTEXPR
-#define FOLLY_STORAGE_CPP14_CONSTEXPR constexpr
-#else
-#define FOLLY_STORAGE_CPP14_CONSTEXPR
-#endif
-#endif
 
 #if __cpp_coroutines >= 201703L && __has_include(<experimental/coroutine>)
 #define FOLLY_HAS_COROUTINES 1
diff --git a/folly/Replaceable.h b/folly/Replaceable.h
index ab6a240a098..2490c18c136 100644
--- a/folly/Replaceable.h
+++ b/folly/Replaceable.h
@@ -439,7 +439,7 @@ class alignas(T) Replaceable
   template <
       class... Args,
       std::enable_if_t<std::is_constructible<T, Args&&...>::value, int> = 0>
-  FOLLY_CPP14_CONSTEXPR explicit Replaceable(in_place_t, Args&&... args)
+  constexpr explicit Replaceable(in_place_t, Args&&... args)
       // clang-format off
       noexcept(std::is_nothrow_constructible<T, Args&&...>::value)
       // clang-format on
@@ -453,7 +453,7 @@ class alignas(T) Replaceable
       std::enable_if_t<
           std::is_constructible<T, std::initializer_list<U>, Args&&...>::value,
           int> = 0>
-  FOLLY_CPP14_CONSTEXPR explicit Replaceable(
+  constexpr explicit Replaceable(
       in_place_t,
       std::initializer_list<U> il,
       Args&&... args)
@@ -475,7 +475,7 @@ class alignas(T) Replaceable
               !std::is_same<Replaceable<T>, std::decay_t<U>>::value &&
               std::is_convertible<U&&, T>::value,
           int> = 0>
-  FOLLY_CPP14_CONSTEXPR /* implicit */ Replaceable(U&& other)
+  constexpr /* implicit */ Replaceable(U&& other)
       // clang-format off
       noexcept(std::is_nothrow_constructible<T, U&&>::value)
       // clang-format on
@@ -491,7 +491,7 @@ class alignas(T) Replaceable
               !std::is_same<Replaceable<T>, std::decay_t<U>>::value &&
               !std::is_convertible<U&&, T>::value,
           int> = 0>
-  FOLLY_CPP14_CONSTEXPR explicit Replaceable(U&& other)
+  constexpr explicit Replaceable(U&& other)
       // clang-format off
       noexcept(std::is_nothrow_constructible<T, U&&>::value)
       // clang-format on
@@ -611,7 +611,7 @@ class alignas(T) Replaceable
     return launder(reinterpret_cast<T const*>(storage_));
   }
 
-  FOLLY_CPP14_CONSTEXPR T* operator->() {
+  constexpr T* operator->() {
     return launder(reinterpret_cast<T*>(storage_));
   }
 
@@ -619,11 +619,11 @@ class alignas(T) Replaceable
     return *launder(reinterpret_cast<T const*>(storage_));
   }
 
-  FOLLY_CPP14_CONSTEXPR T& operator*() & {
+  constexpr T& operator*() & {
     return *launder(reinterpret_cast<T*>(storage_));
   }
 
-  FOLLY_CPP14_CONSTEXPR T&& operator*() && {
+  constexpr T&& operator*() && {
     return std::move(*launder(reinterpret_cast<T*>(storage_)));
   }
 
diff --git a/folly/container/Foreach-inl.h b/folly/container/Foreach-inl.h
index d69d79c7305..792c889d86f 100644
--- a/folly/container/Foreach-inl.h
+++ b/folly/container/Foreach-inl.h
@@ -313,7 +313,7 @@ decltype(auto) fetch_impl(RangeTag, Sequence&& sequence, Index&& index) {
 } // namespace for_each_detail
 
 template <typename Sequence, typename Func>
-FOLLY_CPP14_CONSTEXPR Func for_each(Sequence&& sequence, Func func) {
+constexpr Func for_each(Sequence&& sequence, Func func) {
   namespace fed = for_each_detail;
   using tag = fed::SequenceTag<Sequence>;
   fed::for_each_impl(tag{}, std::forward<Sequence>(sequence), func);
@@ -321,7 +321,7 @@ FOLLY_CPP14_CONSTEXPR Func for_each(Sequence&& sequence, Func func) {
 }
 
 template <typename Sequence, typename Index>
-FOLLY_CPP14_CONSTEXPR decltype(auto) fetch(Sequence&& sequence, Index&& index) {
+constexpr decltype(auto) fetch(Sequence&& sequence, Index&& index) {
   namespace fed = for_each_detail;
   using tag = fed::SequenceTag<Sequence>;
   return for_each_detail::fetch_impl(
diff --git a/folly/container/Foreach.h b/folly/container/Foreach.h
index 3c84128c076..f18b1e337d9 100644
--- a/folly/container/Foreach.h
+++ b/folly/container/Foreach.h
@@ -83,7 +83,7 @@ namespace folly {
  *  });
  */
 template <typename Range, typename Func>
-FOLLY_CPP14_CONSTEXPR Func for_each(Range&& range, Func func);
+constexpr Func for_each(Range&& range, Func func);
 
 /**
  * The user should return loop_break and loop_continue if they want to iterate
@@ -119,7 +119,7 @@ constexpr auto loop_continue = for_each_detail::LoopControl::CONTINUE;
  * required element.
  */
 template <typename Sequence, typename Index>
-FOLLY_CPP14_CONSTEXPR decltype(auto) fetch(Sequence&& sequence, Index&& index);
+constexpr decltype(auto) fetch(Sequence&& sequence, Index&& index);
 
 } // namespace folly
 
diff --git a/folly/lang/PropagateConst.h b/folly/lang/PropagateConst.h
index 539c9189cab..6ddfac305fb 100644
--- a/folly/lang/PropagateConst.h
+++ b/folly/lang/PropagateConst.h
@@ -64,7 +64,7 @@ class propagate_const {
       std::remove_reference_t<decltype(*std::declval<Pointer&>())>;
 
   constexpr propagate_const() = default;
-  FOLLY_CPP14_CONSTEXPR propagate_const(propagate_const&&) = default;
+  constexpr propagate_const(propagate_const&&) = default;
   propagate_const(propagate_const const&) = delete;
 
   template <
@@ -105,14 +105,14 @@ class propagate_const {
   constexpr propagate_const(OtherPointer&& other)
       : pointer_(static_cast<OtherPointer&&>(other)) {}
 
-  FOLLY_CPP14_CONSTEXPR propagate_const& operator=(propagate_const&&) = default;
+  constexpr propagate_const& operator=(propagate_const&&) = default;
   propagate_const& operator=(propagate_const const&) = delete;
 
   template <
       typename OtherPointer,
       typename =
           std::enable_if_t<std::is_convertible<OtherPointer&&, Pointer>::value>>
-  FOLLY_CPP14_CONSTEXPR propagate_const& operator=(
+  constexpr propagate_const& operator=(
       propagate_const<OtherPointer>&& other) {
     pointer_ = static_cast<OtherPointer&&>(other.pointer_);
   }
@@ -122,19 +122,19 @@ class propagate_const {
       typename = std::enable_if_t<
           !detail::is_decay_propagate_const<OtherPointer>::value &&
           std::is_convertible<OtherPointer&&, Pointer>::value>>
-  FOLLY_CPP14_CONSTEXPR propagate_const& operator=(OtherPointer&& other) {
+  constexpr propagate_const& operator=(OtherPointer&& other) {
     pointer_ = static_cast<OtherPointer&&>(other);
     return *this;
   }
 
-  FOLLY_CPP14_CONSTEXPR void swap(propagate_const& other) noexcept(
+  constexpr void swap(propagate_const& other) noexcept(
       noexcept(detail::propagate_const_adl::adl_swap(
           std::declval<Pointer&>(),
           other.pointer_))) {
     detail::propagate_const_adl::adl_swap(pointer_, other.pointer_);
   }
 
-  FOLLY_CPP14_CONSTEXPR element_type* get() {
+  constexpr element_type* get() {
     return get_(pointer_);
   }
 
@@ -146,7 +146,7 @@ class propagate_const {
     return static_cast<bool>(pointer_);
   }
 
-  FOLLY_CPP14_CONSTEXPR element_type& operator*() {
+  constexpr element_type& operator*() {
     return *get();
   }
 
@@ -154,7 +154,7 @@ class propagate_const {
     return *get();
   }
 
-  FOLLY_CPP14_CONSTEXPR element_type* operator->() {
+  constexpr element_type* operator->() {
     return get();
   }
 
@@ -167,7 +167,7 @@ class propagate_const {
       typename = std::enable_if_t<
           std::is_pointer<OtherPointer>::value ||
           std::is_convertible<OtherPointer, element_type*>::value>>
-  FOLLY_CPP14_CONSTEXPR operator element_type*() {
+  constexpr operator element_type*() {
     return get();
   }
 
@@ -199,7 +199,7 @@ class propagate_const {
 };
 
 template <typename Pointer>
-FOLLY_CPP14_CONSTEXPR void swap(
+constexpr void swap(
     propagate_const<Pointer>& a,
     propagate_const<Pointer>& b) noexcept(noexcept(a.swap(b))) {
   a.swap(b);
diff --git a/folly/test/FixedStringTest.cpp b/folly/test/FixedStringTest.cpp
index 056dd909aa0..0f0d98054bd 100644
--- a/folly/test/FixedStringTest.cpp
+++ b/folly/test/FixedStringTest.cpp
@@ -159,7 +159,6 @@ TEST(FixedStringConcatTest, FromTwoStrings) {
   static_assert(res == "hello world!!!", "");
 }
 
-#if FOLLY_USE_CPP14_CONSTEXPR
 constexpr folly::FixedString<20> constexpr_swap_test() {
   folly::FixedString<10> tmp1{"hello"}, tmp2{"world!"};
   tmp2.swap(tmp1);
@@ -169,7 +168,6 @@ constexpr folly::FixedString<20> constexpr_swap_test() {
 TEST(FixedStringSwapTest, ConstexprSwap) {
   static_assert(constexpr_swap_test() == "world!hello", "");
 }
-#endif
 
 TEST(FixedStringSwapTest, RuntimeSwap) {
   folly::FixedString<10> tmp1{"hello"}, tmp2{"world!"};
@@ -177,7 +175,6 @@ TEST(FixedStringSwapTest, RuntimeSwap) {
   EXPECT_STREQ((tmp1 + tmp2).c_str(), "world!hello");
 }
 
-#if FOLLY_USE_CPP14_CONSTEXPR
 constexpr folly::FixedString<10> constexpr_assign_string_test_1() {
   folly::FixedString<10> tmp1, tmp2{"world!"};
   tmp1 = tmp2;
@@ -205,7 +202,6 @@ TEST(FixedStringAssignTest, ConstexprAssignString) {
   static_assert(constexpr_assign_string_test_3() == "db", "");
   static_assert(constexpr_assign_string_test_4() == "dbye", "");
 }
-#endif
 
 TEST(FixedStringAssignTest, RuntimeAssignString) {
   folly::FixedString<10> tmp1, tmp2{"world!"};
@@ -219,7 +215,6 @@ TEST(FixedStringAssignTest, RuntimeAssignString) {
   EXPECT_STREQ("dby", tmp1.c_str());
 }
 
-#if FOLLY_USE_CPP14_CONSTEXPR
 constexpr folly::FixedString<10> constexpr_assign_literal_test_1() {
   folly::FixedString<10> tmp{"aaaaaaaaaa"};
   tmp = "hello";
@@ -244,7 +239,6 @@ TEST(FixedStringAssignTest, ConstexprAssignLiteral) {
   static_assert(constexpr_assign_literal_test_2() == "hello", "");
   static_assert(constexpr_assign_literal_test_3() == "good", "");
 }
-#endif
 
 TEST(FixedStringAssignTest, RuntimeAssignLiteral) {
   folly::FixedString<10> tmp{"aaaaaaaaaa"};
@@ -333,7 +327,6 @@ TEST(FixedStringCompareTest, CompareStdString) {
   EXPECT_TRUE(tmp2 >= tmp1);
 }
 
-#if FOLLY_USE_CPP14_CONSTEXPR
 constexpr folly::FixedString<20> constexpr_append_string_test() {
   folly::FixedString<20> a{"hello"}, b{"X world!"};
   a.append(1u, ' ');
@@ -345,7 +338,6 @@ constexpr folly::FixedString<20> constexpr_append_string_test() {
 TEST(FixedStringAssignTest, ConstexprAppendString) {
   static_assert(constexpr_append_string_test() == "hello world!", "");
 }
-#endif
 
 TEST(FixedStringAssignTest, RuntimeAppendString) {
   folly::FixedString<20> a{"hello"}, b{"X world!"};
@@ -355,7 +347,6 @@ TEST(FixedStringAssignTest, RuntimeAppendString) {
   EXPECT_STREQ("hello world!", a.c_str());
 }
 
-#if FOLLY_USE_CPP14_CONSTEXPR
 constexpr folly::FixedString<20> constexpr_append_literal_test() {
   folly::FixedString<20> a{"hello"};
   a.append(1u, ' ');
@@ -367,7 +358,6 @@ constexpr folly::FixedString<20> constexpr_append_literal_test() {
 TEST(FixedStringAssignTest, ConstexprAppendLiteral) {
   static_assert(constexpr_append_literal_test() == "hello world!", "");
 }
-#endif
 
 TEST(FixedStringAssignTest, RuntimeAppendLiteral) {
   folly::FixedString<20> a{"hello"};
@@ -393,7 +383,6 @@ TEST(FixedStringCAppendTest, CAppendLiteral) {
   static_assert(tmp3 == "hello world!", "");
 }
 
-#if FOLLY_USE_CPP14_CONSTEXPR
 constexpr folly::FixedString<10> constexpr_replace_string_test() {
   folly::FixedString<10> tmp{"abcdefghij"};
   tmp.replace(1, 5, FS("XX"));
@@ -404,7 +393,6 @@ TEST(FixedStringReplaceTest, ConstexprReplaceString) {
   static_assert(constexpr_replace_string_test().size() == 7u, "");
   static_assert(constexpr_replace_string_test() == "aXXghij", "");
 }
-#endif
 
 TEST(FixedStringReplaceTest, RuntimeReplaceString) {
   folly::FixedString<10> tmp{"abcdefghij"};
@@ -637,7 +625,6 @@ TEST(FixedStringConversionTest, ConversionToStdString) {
   EXPECT_STREQ("another string", str.c_str());
 }
 
-#if FOLLY_USE_CPP14_CONSTEXPR
 constexpr std::size_t countSpacesReverse(folly::FixedString<50> s) {
   std::size_t count = 0u;
   auto i = s.rbegin();
@@ -652,7 +639,6 @@ constexpr std::size_t countSpacesReverse(folly::FixedString<50> s) {
 TEST(FixedStringReverseIteratorTest, Cpp14ConstexprReverseIteration) {
   static_assert(3 == countSpacesReverse("This is a string"), "");
 }
-#endif
 
 TEST(FixedStringReverseIteratorTest, ConstexprReverseIteration) {
   static constexpr auto alpha = FS("abcdefghijklmnopqrstuvwxyz");