Fix "undefined reference to `fmt::v7::detail::basic_data<void>::digits'"

include/fmt/core.h

@@ -253,9 +253,10 @@
 #  endif
 #else
 #  define FMT_CLASS_API
-#  if defined(__GNUC__) || defined(__clang__)
-#    define FMT_API __attribute__((visibility("default")))
-#    define FMT_EXTERN_TEMPLATE_API FMT_API
+#  if defined(FMT_EXPORT) || defined(FMT_SHARED)
+#    if defined(__GNUC__) || defined(__clang__)
+#      define FMT_API __attribute__((visibility("default")))
+#    endif
 #  endif
 #endif
 #ifndef FMT_API

include/fmt/format.h

@@ -903,7 +903,7 @@ template <typename T = void> struct basic_data {
   FMT_API static constexpr const char right_padding_shifts[] = {0, 31, 0, 1, 0};
 };
 
-#ifndef FMT_HEADER_ONLY
+#ifdef FMT_SHARED
 // Required for -flto, -fivisibility=hidden and -shared to work
 extern template struct basic_data<void>;
 #endif

test/CMakeLists.txt

@@ -213,6 +213,21 @@ if (FMT_PEDANTIC AND NOT WIN32)
     "-DCMAKE_BUILD_TYPE=${CMAKE_BUILD_TYPE}")
 endif ()
 
+# This test are disabled on Windows because it is only *NIX issue.
+if (FMT_PEDANTIC AND NOT WIN32)
+  add_test(static-export-test ${CMAKE_CTEST_COMMAND}
+    -C ${CMAKE_BUILD_TYPE}
+    --build-and-test
+    "${CMAKE_CURRENT_SOURCE_DIR}/static-export-test"
+    "${CMAKE_CURRENT_BINARY_DIR}/static-export-test"
+    --build-generator ${CMAKE_GENERATOR}
+    --build-makeprogram ${CMAKE_MAKE_PROGRAM}
+    --build-options
+    "-DCMAKE_CXX_COMPILER=${CMAKE_CXX_COMPILER}"
+    "-DCMAKE_CXX_STANDARD=${CMAKE_CXX_STANDARD}"
+    "-DCMAKE_BUILD_TYPE=${CMAKE_BUILD_TYPE}")
+endif ()
+
 # Activate optional CUDA tests if CUDA is found. For version selection see
 # https://docs.nvidia.com/cuda/cuda-c-programming-guide/index.html#cpp14-language-features
 if (FMT_CUDA_TEST)

test/static-export-test/CMakeLists.txt

@@ -0,0 +1,30 @@
+cmake_minimum_required(VERSION 3.1...3.18)
+
+project(fmt-link CXX)
+
+set(BUILD_SHARED_LIBS OFF)
+set(CMAKE_VISIBILITY_INLINES_HIDDEN TRUE)
+set(CMAKE_CXX_VISIBILITY_PRESET "hidden")
+
+# Broken LTO on GCC 4
+if (CMAKE_COMPILER_IS_GNUCXX AND CMAKE_CXX_COMPILER_VERSION VERSION_LESS 5)
+  set(BROKEN_LTO ON)
+endif ()
+
+if (NOT BROKEN_LTO AND CMAKE_VERSION VERSION_GREATER "3.8")
+  # CMake 3.9+
+  include(CheckIPOSupported)
+  check_ipo_supported(RESULT HAVE_IPO)
+  if (HAVE_IPO)
+    set(CMAKE_INTERPROCEDURAL_OPTIMIZATION TRUE)
+  endif ()
+endif ()
+
+add_subdirectory(../.. fmt)
+set_property(TARGET fmt PROPERTY POSITION_INDEPENDENT_CODE ON)
+
+add_library(library-test SHARED library.cc)
+target_link_libraries(library-test PRIVATE fmt::fmt)
+
+add_executable(exe-test main.cc)
+target_link_libraries(exe-test PRIVATE library-test)

test/static-export-test/library.cc

@@ -0,0 +1,5 @@
+#include <fmt/compile.h>
+
+__attribute__((visibility("default"))) std::string foo() {
+  return fmt::format(FMT_COMPILE("foo bar {}"), 4242);
+}

test/static-export-test/main.cc

@@ -0,0 +1,6 @@
+#include <iostream>
+#include <string>
+
+extern std::string foo();
+
+int main() { std::cout << foo() << std::endl; }