Improve support for non-POSIX platforms more

include/fmt/format-inl.h

@@ -1443,17 +1443,24 @@ template <typename T> struct span {
   size_t size;
 };
 
-#ifdef _WIN32
-inline void flockfile(FILE* f) { _lock_file(f); }
-inline void funlockfile(FILE* f) { _unlock_file(f); }
-inline int getc_unlocked(FILE* f) { return _fgetc_nolock(f); }
+template <typename F> auto flockfile(F* f) -> decltype(_lock_file(f)) {
+  _lock_file(f);
+}
+template <typename F> auto funlockfile(F* f) -> decltype(_unlock_file(f)) {
+  _unlock_file(f);
+}
+
+#ifndef getc_unlocked
+template <typename F> auto getc_unlocked(F* f) -> decltype(_fgetc_nolock(f)) {
+  return _fgetc_nolock(f);
+}
 #endif
 
 template <typename F = FILE, typename Enable = void>
 struct has_flockfile : std::false_type {};
 
 template <typename F>
-struct has_flockfile<F, void_t<decltype(flockfile(static_cast<F*>(nullptr)))>>
+struct has_flockfile<F, void_t<decltype(flockfile(&std::declval<F&>()))>>
     : std::true_type {};
 
 // A FILE wrapper. F is FILE defined as a template parameter to make system API
@@ -1658,7 +1665,8 @@ class file_print_buffer<F, enable_if_t<has_flockfile<F>::value>>
   ~file_print_buffer() {
     file_.advance_write_buffer(size());
     bool flush = file_.needs_flush();
-    funlockfile(file_);
+    F* f = file_;    // Make funlockfile depend on the template parameter F
+    funlockfile(f);  // for the system API detection to work.
     if (flush) fflush(file_);
   }
 };

test/scan.h

@@ -192,7 +192,10 @@ class file_scan_buffer final : public scan_buffer {
     flockfile(f);
     fill();
   }
-  ~file_scan_buffer() { funlockfile(file_); }
+  ~file_scan_buffer() {
+    FILE* f = file_;
+    funlockfile(f);
+  }
 };
 }  // namespace detail