add compile-time formatting switch and empty test

include/fmt/core.h

@@ -252,6 +252,17 @@
 #  pragma execution_character_set("utf-8")
 #endif
 
+#if defined(FMT_ENABLE_COMPILE_TIME_FORMATTING)
+#  if !defined(FMT_HEADER_ONLY) || __cplusplus < 202002L
+#    error "compile time formatting could not be enabled"
+#  endif
+#  define FMT_CTF_CONSTEXPR constexpr
+#  define FMT_USE_CUSTOM_BACK_INSERT_ITERATOR 1
+#else
+#  define FMT_CTF_CONSTEXPR
+#  define FMT_USE_CUSTOM_BACK_INSERT_ITERATOR 0
+#endif
+
 FMT_BEGIN_NAMESPACE
 
 // Implementations of enable_if_t and other metafunctions for older systems.

test/CMakeLists.txt

@@ -146,6 +146,12 @@ else ()
   target_compile_definitions(header-only-test PRIVATE FMT_HEADER_ONLY=1)
 endif ()
 
+add_fmt_executable(compile-time-formatting-test
+  compile-time-formatting-test.cc test-main.cc)
+target_link_libraries(compile-time-formatting-test gmock)
+target_include_directories(compile-time-formatting-test SYSTEM PUBLIC gtest gmock)
+target_link_libraries(compile-time-formatting-test fmt::fmt-header-only)
+
 message(STATUS "FMT_PEDANTIC: ${FMT_PEDANTIC}")
 
 if (FMT_PEDANTIC)

test/compile-time-formatting-test.cc

@@ -0,0 +1,21 @@
+#include <array>
+#include <string>
+
+#define FMT_ENABLE_COMPILE_TIME_FORMATTING 1
+#include "fmt/format.h"
+#include "gmock.h"
+#include "gtest-extra.h"
+
+template <size_t max_string_length> struct constexpr_buffer_helper {
+  template <typename Func>
+  constexpr constexpr_buffer_helper& modify(Func func) {
+    func(buffer);
+    return *this;
+  }
+
+  template <typename T> constexpr bool operator==(const T& rhs) const noexcept {
+    return (std::string_view(rhs).compare(buffer.data()) == 0);
+  }
+
+  std::array<char, max_string_length> buffer{};
+};