Remove problematic constructibility check

include/fmt/core.h

@@ -1337,20 +1337,19 @@ template <typename Context> struct arg_mapper {
   }
   template <typename T,
             FMT_ENABLE_IF(
-                std::is_constructible<basic_string_view<char_type>, T>::value &&
+                std::is_convertible<T, basic_string_view<char_type>>::value &&
                 !is_string<T>::value && !has_formatter<T, Context>::value &&
                 !has_fallback_formatter<T, char_type>::value)>
   FMT_CONSTEXPR FMT_INLINE auto map(const T& val)
       -> basic_string_view<char_type> {
     return basic_string_view<char_type>(val);
   }
-  template <
-      typename T,
-      FMT_ENABLE_IF(
-          std::is_constructible<std_string_view<char_type>, T>::value &&
-          !std::is_constructible<basic_string_view<char_type>, T>::value &&
-          !is_string<T>::value && !has_formatter<T, Context>::value &&
-          !has_fallback_formatter<T, char_type>::value)>
+  template <typename T,
+            FMT_ENABLE_IF(
+                std::is_convertible<T, std_string_view<char_type>>::value &&
+                !std::is_convertible<T, basic_string_view<char_type>>::value &&
+                !is_string<T>::value && !has_formatter<T, Context>::value &&
+                !has_fallback_formatter<T, char_type>::value)>
   FMT_CONSTEXPR FMT_INLINE auto map(const T& val)
       -> basic_string_view<char_type> {
     return std_string_view<char_type>(val);

test/core-test.cc

@@ -890,7 +890,10 @@ struct explicitly_convertible_to_string_view {
 };
 
 TEST(core_test, format_explicitly_convertible_to_string_view) {
-  EXPECT_EQ("foo", fmt::format("{}", explicitly_convertible_to_string_view()));
+  // Types explicitly convertible to string_view are not formattable by
+  // default because it may introduce ODR violations.
+  static_assert(
+      !fmt::is_formattable<explicitly_convertible_to_string_view>::value, "");
 }
 
 #  ifdef FMT_USE_STRING_VIEW
@@ -899,8 +902,11 @@ struct explicitly_convertible_to_std_string_view {
 };
 
 TEST(core_test, format_explicitly_convertible_to_std_string_view) {
-  EXPECT_EQ("foo",
-            fmt::format("{}", explicitly_convertible_to_std_string_view()));
+  // Types explicitly convertible to string_view are not formattable by
+  // default because it may introduce ODR violations.
+  static_assert(
+      !fmt::is_formattable<explicitly_convertible_to_std_string_view>::value,
+      "");
 }
 #  endif
 #endif

test/ranges-test.cc

@@ -133,8 +133,8 @@ TEST(ranges_test, path_like) {
 struct string_like {
   const char* begin();
   const char* end();
-  explicit operator fmt::string_view() const { return "foo"; }
-  explicit operator std::string_view() const { return "foo"; }
+  operator fmt::string_view() const { return "foo"; }
+  operator std::string_view() const { return "foo"; }
 };
 
 TEST(ranges_test, format_string_like) {

test/xchar-test.cc

@@ -72,8 +72,10 @@ struct explicitly_convertible_to_wstring_view {
 };
 
 TEST(xchar_test, format_explicitly_convertible_to_wstring_view) {
-  EXPECT_EQ(L"foo",
-            fmt::format(L"{}", explicitly_convertible_to_wstring_view()));
+  // Types explicitly convertible to wstring_view are not formattable by
+  // default because it may introduce ODR violations.
+  static_assert(
+      !fmt::is_formattable<explicitly_convertible_to_wstring_view>::value, "");
 }
 #endif