Fix handling of formattable types with to_string_view (#2181)

include/fmt/format.h

@@ -203,8 +203,7 @@ FMT_END_NAMESPACE
 // Some compilers masquerade as both MSVC and GCC-likes or otherwise support
 // __builtin_clz and __builtin_clzll, so only define FMT_BUILTIN_CLZ using the
 // MSVC intrinsics if the clz and clzll builtins are not available.
-#if FMT_MSC_VER && !defined(FMT_BUILTIN_CLZLL) && \
-    !defined(FMT_BUILTIN_CTZLL)
+#if FMT_MSC_VER && !defined(FMT_BUILTIN_CLZLL) && !defined(FMT_BUILTIN_CTZLL)
 FMT_BEGIN_NAMESPACE
 namespace detail {
 // Avoid Clang with Microsoft CodeGen's -Wunknown-pragmas warning.
@@ -2103,6 +2102,12 @@ FMT_CONSTEXPR OutputIt write(OutputIt out, basic_string_view<Char> value) {
   return base_iterator(out, it);
 }
 
+template <typename Char, typename OutputIt, typename T,
+          FMT_ENABLE_IF(is_string<T>::value)>
+constexpr OutputIt write(OutputIt out, const T& value) {
+  return write<Char>(out, to_string_view(value));
+}
+
 template <typename Char, typename OutputIt, typename T,
           FMT_ENABLE_IF(is_integral<T>::value &&
                         !std::is_same<T, bool>::value &&

test/compile-test.cc

@@ -288,6 +288,27 @@ TEST(CompileTest, UnknownFormatFallback) {
 }
 
 TEST(CompileTest, Empty) { EXPECT_EQ("", fmt::format(FMT_COMPILE(""))); }
+
+struct to_stringable {
+  friend fmt::string_view to_string_view(to_stringable) { return {}; }
+};
+
+FMT_BEGIN_NAMESPACE
+template <> struct formatter<to_stringable> {
+  auto parse(format_parse_context& ctx) const -> decltype(ctx.begin()) {
+    return ctx.begin();
+  }
+
+  template <typename FormatContext>
+  auto format(const to_stringable&, FormatContext& ctx) -> decltype(ctx.out()) {
+    return ctx.out();
+  }
+};
+FMT_END_NAMESPACE
+
+TEST(CompileTest, ToStringAndFormatter) {
+  fmt::format(FMT_COMPILE("{}"), to_stringable());
+}
 #endif
 
 #if FMT_USE_NONTYPE_TEMPLATE_PARAMETERS