Fix #4303: avoid instantiating formatter<const T> (#4325)

Author: timsong-cpp

include/fmt/base.h

@@ -1121,7 +1121,7 @@ using use_formatter =
     bool_constant<(std::is_class<T>::value || std::is_enum<T>::value ||
                    std::is_union<T>::value || std::is_array<T>::value) &&
                   !has_to_string_view<T>::value && !is_named_arg<T>::value &&
-                  !use_format_as<T>::value && !use_format_as_member<T>::value>;
+                  !use_format_as<T>::value && !use_format_as_member<U>::value>;
 
 template <typename Char, typename T, typename U = remove_const_t<T>>
 auto has_formatter_impl(T* p, buffered_context<Char>* ctx = nullptr)

test/std-test.cc

@@ -13,13 +13,17 @@
 #include <vector>
 
 #include "fmt/os.h"  // fmt::system_category
-#include "fmt/ranges.h"
 #include "gtest-extra.h"  // StartsWith
 
 #ifdef __cpp_lib_filesystem
 TEST(std_test, path) {
   using std::filesystem::path;
   EXPECT_EQ(fmt::format("{}", path("/usr/bin")), "/usr/bin");
+
+  // see #4303
+  const path p = "/usr/bin";
+  EXPECT_EQ(fmt::format("{}", p), "/usr/bin");
+
   EXPECT_EQ(fmt::format("{:?}", path("/usr/bin")), "\"/usr/bin\"");
   EXPECT_EQ(fmt::format("{:8}", path("foo")), "foo     ");
 
@@ -44,6 +48,9 @@ TEST(std_test, path) {
 #  endif
 }
 
+// Intentionally delayed include to test #4303
+#include "fmt/ranges.h"
+
 // Test ambiguity problem described in #2954.
 TEST(ranges_std_test, format_vector_path) {
   auto p = std::filesystem::path("foo/bar.txt");