Add support for incomplete types

test/format-test.cc

@@ -2540,3 +2540,29 @@ TEST(base_test, format_byte) {
   EXPECT_EQ(s, "42");
 }
 #endif
+
+// Only defined after the test case.
+struct incomplete_type;
+extern const incomplete_type& external_instance;
+
+FMT_BEGIN_NAMESPACE
+template <> struct formatter<incomplete_type> : formatter<int> {
+  auto format(const incomplete_type& x, context& ctx) const -> appender;
+};
+FMT_END_NAMESPACE
+
+TEST(incomplete_type_test, format) {
+  EXPECT_EQ(fmt::format("{}", external_instance), "42");
+}
+
+struct incomplete_type {
+  int i;
+};
+
+const incomplete_type& external_instance = {42};
+
+auto fmt::formatter<incomplete_type>::format(const incomplete_type& x,
+                                             fmt::context& ctx) const
+    -> fmt::appender {
+  return formatter<int>::format(x.i, ctx);
+}