Make ostream formatter work with compile-time format strings (#1692)

include/fmt/ostream.h

@@ -110,7 +110,8 @@ void format_value(buffer<Char>& buf, const T& value,
 template <typename T, typename Char>
 struct fallback_formatter<T, Char, enable_if_t<is_streamable<T, Char>::value>>
     : private formatter<basic_string_view<Char>, Char> {
-  auto parse(basic_format_parse_context<Char>& ctx) -> decltype(ctx.begin()) {
+  FMT_CONSTEXPR auto parse(basic_format_parse_context<Char>& ctx)
+      -> decltype(ctx.begin()) {
     return formatter<basic_string_view<Char>, Char>::parse(ctx);
   }
   template <typename ParseCtx,

test/ostream-test.cc

@@ -302,3 +302,7 @@ std::ostream& operator<<(std::ostream& os, copyfmt_test) {
 TEST(OStreamTest, CopyFmt) {
   EXPECT_EQ("foo", fmt::format("{}", copyfmt_test()));
 }
+
+TEST(OStreamTest, CompileTimeString) {
+  EXPECT_EQ("42", fmt::format(FMT_STRING("{}"), 42));
+}