Make compile-time checks work with fallback formatter (#1088)

include/fmt/core.h

@@ -674,6 +674,15 @@ template <typename Context> struct custom_value {
                  Context& ctx);
 };
 
+template <typename T, typename Context>
+struct is_formattable {
+  enum {
+    value = !is_constructible<
+      typename Context::template formatter_type<T>::type,
+      internal::dummy_formatter_arg>::value
+  };
+};
+
 // A formatting argument value.
 template <typename Context> class value {
  public:
@@ -721,14 +730,10 @@ template <typename Context> class value {
     // Get the formatter type through the context to allow different contexts
     // have different extension points, e.g. `formatter<T>` for `format` and
     // `printf_formatter<T>` for `printf`.
-    typedef typename Context::template formatter_type<T>::type formatter;
-    enum {
-      formattable =
-          !is_constructible<formatter, internal::dummy_formatter_arg>::value
-    };
     custom.format = &format_custom_arg<
         T, typename std::conditional<
-               formattable, formatter,
+               is_formattable<T, Context>::value,
+               typename Context::template formatter_type<T>::type,
                internal::fallback_formatter<T, char_type>>::type>;
   }
 

include/fmt/format.h

@@ -2136,7 +2136,10 @@ template <typename T, typename ParseContext>
 FMT_CONSTEXPR const typename ParseContext::char_type* parse_format_specs(
     ParseContext& ctx) {
   // GCC 7.2 requires initializer.
-  formatter<T, typename ParseContext::char_type> f{};
+  typedef typename ParseContext::char_type char_type;
+  typename std::conditional<
+    is_formattable<T, format_context>::value,
+    formatter<T, char_type>, internal::fallback_formatter<T, char_type>>::type f;
   return f.parse(ctx);
 }
 

test/ostream-test.cc

@@ -173,6 +173,7 @@ TEST(OStreamTest, Join) {
 #if FMT_USE_CONSTEXPR
 TEST(OStreamTest, ConstexprString) {
   EXPECT_EQ("42", format(fmt("{}"), std::string("42")));
+  EXPECT_EQ("a string", format(fmt("{0}"), TestString("a string")));
 }
 #endif