Make FMT_COMPILE fallback on runtime without if constexpr (#2261)

Author: vitaut

include/fmt/compile.h

@@ -114,7 +114,11 @@ struct is_compiled_string : std::is_base_of<compiled_string, S> {};
     std::string s = fmt::format(FMT_COMPILE("{}"), 42);
   \endrst
  */
-#define FMT_COMPILE(s) FMT_STRING_IMPL(s, fmt::detail::compiled_string)
+#ifdef __cpp_if_constexpr
+#  define FMT_COMPILE(s) FMT_STRING_IMPL(s, fmt::detail::compiled_string)
+#else
+#  define FMT_COMPILE(s) FMT_STRING(s)
+#endif
 
 #if FMT_USE_NONTYPE_TEMPLATE_PARAMETERS
 template <typename Char, size_t N, fixed_string<Char, N> Str>

test/compile-test.cc

@@ -155,6 +155,12 @@ TEST(CompileTest, EmptyFormatString) {
   EXPECT_EQ(fmt::format(f), "");
 }
 
+TEST(CompileTest, CompileFallback) {
+  // FMT_COMPILE should fallback on runtime formatting when `if constexpr` is
+  // not available.
+  EXPECT_EQ("42", fmt::format(FMT_COMPILE("{}"), 42));
+}
+
 #ifdef __cpp_if_constexpr
 TEST(CompileTest, FormatDefault) {
   EXPECT_EQ("42", fmt::format(FMT_COMPILE("{}"), 42));