Fix gcc 7.2 issue

include/fmt/format.h

@@ -1992,7 +1992,8 @@ FMT_CONSTEXPR void parse_format_string(Iterator it, Handler &&handler) {
 template <typename T, typename ParseContext>
 FMT_CONSTEXPR const typename ParseContext::char_type *
     parse_format_specs(ParseContext &ctx) {
-  formatter<T, typename ParseContext::char_type> f;
+  // GCC 7.2 requires initializer.
+  formatter<T, typename ParseContext::char_type> f{};
   return f.parse(ctx);
 }
 

test/format-test.cc

@@ -1929,7 +1929,7 @@ FMT_CONSTEXPR bool test_error(const char *fmt, const char *expected_error) {
 TEST(FormatTest, FormatStringErrors) {
   EXPECT_ERROR("foo", nullptr);
   EXPECT_ERROR("}", "unmatched '}' in format string");
-  //EXPECT_ERROR("{0:s", "unknown format specifier", Date);
+  EXPECT_ERROR("{0:s", "unknown format specifier", Date);
 #ifndef _MSC_VER
   // This causes an internal compiler error in MSVC2017.
   EXPECT_ERROR("{0:=5", "unknown format specifier", int);