Fix clang warning about ignoring __declspec(dllexport) on basic_data<void> template instantitation definition (fmtlib#2220)

Author: denchat

include/fmt/core.h

@@ -223,8 +223,8 @@
 #ifndef FMT_EXTERN_TEMPLATE_API
 #  define FMT_EXTERN_TEMPLATE_API
 #endif
-#ifndef FMT_INSTANTIATION_DEF_API
-#  define FMT_INSTANTIATION_DEF_API FMT_API
+#ifndef FMT_INSTANTIATION_DECL_API
+#  define FMT_INSTANTIATION_DECL_API FMT_API
 #endif
 
 #ifndef FMT_HEADER_ONLY

include/fmt/format.h

@@ -995,9 +995,7 @@ FMT_INLINE uint16_t bsr2log10(int bsr) {
   return data[bsr];
 }
 
-#ifndef FMT_EXPORTED
-FMT_EXTERN template struct basic_data<void>;
-#endif
+FMT_EXTERN template struct FMT_INSTANTIATION_DECL_API basic_data<void>;
 
 // This is a struct rather than an alias to avoid shadowing warnings in gcc.
 struct data : basic_data<> {};

src/format.cc

@@ -55,7 +55,9 @@ vformat_to(buffer<char>&, string_view,
                type_identity_t<char>>>);
 }  // namespace detail
 
-template struct FMT_INSTANTIATION_DEF_API detail::basic_data<void>;
+// Clang doesn't allow dllexport on template instantiation definitions (LLVM
+// D61118).
+template struct detail::basic_data<void>;
 
 // Workaround a bug in MSVC2013 that prevents instantiation of format_float.
 int (*instantiate_format_float)(double, int, detail::float_specs,