From 29c10fbf6e51d2a204f7c24c90d7ccec86cc659a Mon Sep 17 00:00:00 2001
From: denchat <19730041+denchat@users.noreply.github.com>
Date: Thu, 2 May 2019 21:49:01 +0700
Subject: [PATCH] Fix DLL visibility of explicit instantiation "declaration" of
 internal::basic_data<void> in header format.h and the explicit instantiation
 "definition" in format.cc (#1134)

* Update format.cc

As the explicit instantiation *declaration* of `internal::basic_data<void>` in format.h, this explicit instantiation *definition* should mirror FMT_API also.

* Mirror visibility of explicit instantiation declaration

explicit instantiation declaration of internal::basic_data<void> should mirror visibility of FMT_API

* Eliminate `__declspec(dllexport)` designation on extern template internal::basic_data<> when `extern` affected during exporting phase.

* Add `FMT_EXTERN_TEMPLATE_API` for designate DLL export `extern template`

When exporting DLL, do not designate `__declspec(dllexport)` any template that has any explicit class template declaration a.k.a. `extern template`. Instead, designate `__declspec(dllexport)` at single point where we have explicit class template definition a.k.a. normal instantiation without `extern`

Note: this is a c++11 feature.

* Delete whole `FMT_USE_EXTERN_TEMPLATES` block and its condition

1. Remove whole `FMT_USE_EXTERN_TEMPLATES` block
(trailing `FMT_UDL_TEMPLATE` block)
````
#ifndef FMT_USE_EXTERN_TEMPLATES
#  ifndef FMT_HEADER_ONLY
#    define FMT_USE_EXTERN_TEMPLATES                           \
      ((FMT_CLANG_VERSION >= 209 && __cplusplus >= 201103L) || \
       (FMT_GCC_VERSION >= 303 && FMT_HAS_GXX_CXX11))
#  else
#    define FMT_USE_EXTERN_TEMPLATES 0
#  endif
#endif
````

2. Delete `FMT_USE_EXTERN_TEMPLATES` condition, only condition, that trailing basic_data class template definition.
````
#if FMT_USE_EXTERN_TEMPLATES
extern template struct basic_data<void>;
#endif
````

3. Replace `FMT_API` with new `FMT_EXTERN_TEMPLATE_API` added in `core.h` for sake of extern template of `basic_data<void>`

* Add `#define FMT_EXTERN extern` only when not `FMT_HEADER_ONLY`

* Replace `extern` on basic_data<void> with the `FMT_EXTERN` condition in core.h

* replace misspelled if !define() with ifndef
---
 include/fmt/core.h   | 10 ++++++++++
 include/fmt/format.h | 16 ++--------------
 src/format.cc        |  2 +-
 3 files changed, 13 insertions(+), 15 deletions(-)

diff --git a/include/fmt/core.h b/include/fmt/core.h
index 8dc68e40..731fe965 100644
--- a/include/fmt/core.h
+++ b/include/fmt/core.h
@@ -189,11 +189,21 @@
 #    define FMT_API __declspec(dllexport)
 #  elif defined(FMT_SHARED)
 #    define FMT_API __declspec(dllimport)
+#    define FMT_EXTERN_TEMPLATE_API FMT_API
 #  endif
 #endif
 #ifndef FMT_API
 #  define FMT_API
 #endif
+#ifndef FMT_EXTERN_TEMPLATE_API
+#  define FMT_EXTERN_TEMPLATE_API
+#endif
+
+#ifndef FMT_HEADER_ONLY
+#  define FMT_EXTERN extern
+#else
+#  define FMT_EXTERN
+#endif
 
 #ifndef FMT_ASSERT
 #  define FMT_ASSERT(condition, message) assert((condition) && message)
diff --git a/include/fmt/format.h b/include/fmt/format.h
index 8e101c45..52caad57 100644
--- a/include/fmt/format.h
+++ b/include/fmt/format.h
@@ -159,16 +159,6 @@ FMT_END_NAMESPACE
 #  define FMT_UDL_TEMPLATE 0
 #endif
 
-#ifndef FMT_USE_EXTERN_TEMPLATES
-#  ifndef FMT_HEADER_ONLY
-#    define FMT_USE_EXTERN_TEMPLATES                           \
-      ((FMT_CLANG_VERSION >= 209 && __cplusplus >= 201103L) || \
-       (FMT_GCC_VERSION >= 303 && FMT_HAS_GXX_CXX11))
-#  else
-#    define FMT_USE_EXTERN_TEMPLATES 0
-#  endif
-#endif
-
 #if FMT_HAS_GXX_CXX11 || FMT_HAS_FEATURE(cxx_trailing_return) || \
     FMT_MSC_VER >= 1600
 #  define FMT_USE_TRAILING_RETURN 1
@@ -712,7 +702,7 @@ template <typename T> struct int_traits {
 
 // Static data is placed in this class template to allow header-only
 // configuration.
-template <typename T = void> struct FMT_API basic_data {
+template <typename T = void> struct FMT_EXTERN_TEMPLATE_API basic_data {
   static const uint64_t POWERS_OF_10_64[];
   static const uint32_t ZERO_OR_POWERS_OF_10_32[];
   static const uint64_t ZERO_OR_POWERS_OF_10_64[];
@@ -726,9 +716,7 @@ template <typename T = void> struct FMT_API basic_data {
   static const wchar_t WRESET_COLOR[5];
 };
 
-#if FMT_USE_EXTERN_TEMPLATES
-extern template struct basic_data<void>;
-#endif
+FMT_EXTERN template struct basic_data<void>;
 
 // This is a struct rather than a typedef to avoid shadowing warnings in gcc.
 struct data : basic_data<> {};
diff --git a/src/format.cc b/src/format.cc
index 6b268bbe..b60b7e4f 100644
--- a/src/format.cc
+++ b/src/format.cc
@@ -8,7 +8,7 @@
 #include "fmt/format-inl.h"
 
 FMT_BEGIN_NAMESPACE
-template struct internal::basic_data<void>;
+template struct FMT_API internal::basic_data<void>;
 
 // Workaround a bug in MSVC2013 that prevents instantiation of grisu_format.
 bool (*instantiate_grisu_format)(double, internal::buffer<char>&, int, unsigned,