Clang rejects call to base class member function inside requires inside constexpr if using scope resolution operator

[ranaanoop]

The following program is rejected by clang :Demo

#include<type_traits>
  
template<typename T>
struct AlwaysFalse : public std::false_type {};

struct B
{
  template<typename S>
  void foo(){}
};

template <typename T,typename S>
struct A:T
{
  auto foo(){
    if constexpr (requires{T::template foo<S>();})
    return 1;
    else
       static_assert(AlwaysFalse<A>::value,
                    "foo is not implemented.");
  }
};

int main()
{
A<B,double> a;
a.foo();
}

Clang says:

<source>:19:22: error: static assertion failed due to requirement 'AlwaysFalse<A<B, double>>::value': foo is not implemented.
   19 |        static_assert(AlwaysFalse<A>::value,
      |                      ^~~~~~~~~~~~~~~~~~~~~
<source>:27:3: note: in instantiation of member function 'A<B, double>::foo' requested here
   27 | a.foo();
      |   ^
1 error generated.
Compiler returned: 1

---

Note program work if we change constexpr if to if constexpr (requires(T t){t. template foo<S>();}). Demo

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Anoop Rana (ranaanoop)

The following program is rejected by clang :[Demo](https://godbolt.org/z/34ra7aM1s) ``` #include<type_traits>

template<typename T>
struct AlwaysFalse : public std::false_type {};

struct B
{
template<typename S>
void foo(){}
};

template <typename T,typename S>
struct A:T
{
auto foo(){
if constexpr (requires{T::template foo<S>();})
return 1;
else
static_assert(AlwaysFalse<A>::value,
"foo is not implemented.");
}
};

int main()
{
A<B,double> a;
a.foo();
}

Clang says:

<source>:19:22: error: static assertion failed due to requirement 'AlwaysFalse<A<B, double>>::value': foo is not implemented.
19 | static_assert(AlwaysFalse<A>::value,
| ^~~~~~~~~~~~~~~~~~~~~
<source>:27:3: note: in instantiation of member function 'A<B, double>::foo' requested here
27 | a.foo();
| ^
1 error generated.
Compiler returned: 1

---

Note program work if we change constexpr if to `if constexpr (requires(T t){t. template foo<S>();})`. [Demo](https://godbolt.org/z/jKGfenq1o)
</details>

[llvmbot]

@llvm/issue-subscribers-c-20

Author: Anoop Rana (ranaanoop)

The following program is rejected by clang :[Demo](https://godbolt.org/z/34ra7aM1s) ``` #include<type_traits>

template<typename T>
struct AlwaysFalse : public std::false_type {};

struct B
{
template<typename S>
void foo(){}
};

template <typename T,typename S>
struct A:T
{
auto foo(){
if constexpr (requires{T::template foo<S>();})
return 1;
else
static_assert(AlwaysFalse<A>::value,
"foo is not implemented.");
}
};

int main()
{
A<B,double> a;
a.foo();
}

Clang says:

<source>:19:22: error: static assertion failed due to requirement 'AlwaysFalse<A<B, double>>::value': foo is not implemented.
19 | static_assert(AlwaysFalse<A>::value,
| ^~~~~~~~~~~~~~~~~~~~~
<source>:27:3: note: in instantiation of member function 'A<B, double>::foo' requested here
27 | a.foo();
| ^
1 error generated.
Compiler returned: 1

---

Note program work if we change constexpr if to `if constexpr (requires(T t){t. template foo<S>();})`. [Demo](https://godbolt.org/z/jKGfenq1o)
</details>

[shafik]

Duplicate: #84020

When a bug comes from a Stackoverflow we should link to the SO post for reference, this should also help to catch duplicates when screening bugs, thank you!