From 1f1b50707c8f56095481e29594b0da24431f82d5 Mon Sep 17 00:00:00 2001
From: Victor Zverovich <viz@fb.com>
Date: Fri, 7 Feb 2020 18:47:48 -0800
Subject: [PATCH] Make formatter override implicit conversion to a C string

---
 include/fmt/core.h   | 10 +++++-----
 test/core-test.cc    | 15 +++++++++++++++
 test/ostream-test.cc | 18 +++++-------------
 3 files changed, 25 insertions(+), 18 deletions(-)

diff --git a/include/fmt/core.h b/include/fmt/core.h
index 6824c125..35c6414c 100644
--- a/include/fmt/core.h
+++ b/include/fmt/core.h
@@ -900,7 +900,8 @@ template <typename Context> struct arg_mapper {
   template <typename T,
             FMT_ENABLE_IF(
                 std::is_constructible<basic_string_view<char_type>, T>::value &&
-                !is_string<T>::value)>
+                !is_string<T>::value && !has_formatter<T, Context>::value &&
+                !has_fallback_formatter<T, Context>::value)>
   FMT_CONSTEXPR basic_string_view<char_type> map(const T& val) {
     return basic_string_view<char_type>(val);
   }
@@ -909,7 +910,8 @@ template <typename Context> struct arg_mapper {
       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)>
+          !is_string<T>::value && !has_formatter<T, Context>::value &&
+          !has_fallback_formatter<T, Context>::value)>
   FMT_CONSTEXPR basic_string_view<char_type> map(const T& val) {
     return std_string_view<char_type>(val);
   }
@@ -946,10 +948,8 @@ template <typename Context> struct arg_mapper {
       typename T,
       FMT_ENABLE_IF(
           !is_string<T>::value && !is_char<T>::value &&
-          !std::is_constructible<basic_string_view<char_type>, T>::value &&
           (has_formatter<T, Context>::value ||
-           (has_fallback_formatter<T, Context>::value &&
-            !std::is_constructible<std_string_view<char_type>, T>::value)))>
+           has_fallback_formatter<T, Context>::value))>
   FMT_CONSTEXPR const T& map(const T& val) {
     return val;
   }
diff --git a/test/core-test.cc b/test/core-test.cc
index 8f233ece..6a7c0d3b 100644
--- a/test/core-test.cc
+++ b/test/core-test.cc
@@ -469,6 +469,10 @@ struct convertible_to_int {
   operator int() const { return 42; }
 };
 
+struct convertible_to_c_string {
+  operator const char*() const { return "foo"; }
+};
+
 FMT_BEGIN_NAMESPACE
 template <> struct formatter<convertible_to_int> {
   auto parse(format_parse_context& ctx) -> decltype(ctx.begin()) {
@@ -478,10 +482,21 @@ template <> struct formatter<convertible_to_int> {
     return std::copy_n("foo", 3, ctx.out());
   }
 };
+
+template <> struct formatter<convertible_to_c_string> {
+  auto parse(format_parse_context& ctx) -> decltype(ctx.begin()) {
+    return ctx.begin();
+  }
+  auto format(convertible_to_c_string, format_context& ctx)
+      -> decltype(ctx.out()) {
+    return std::copy_n("bar", 3, ctx.out());
+  }
+};
 FMT_END_NAMESPACE
 
 TEST(CoreTest, FormatterOverridesImplicitConversion) {
   EXPECT_EQ(fmt::format("{}", convertible_to_int()), "foo");
+  EXPECT_EQ(fmt::format("{}", convertible_to_c_string()), "bar");
 }
 
 namespace my_ns {
diff --git a/test/ostream-test.cc b/test/ostream-test.cc
index 9944618e..d9329345 100644
--- a/test/ostream-test.cc
+++ b/test/ostream-test.cc
@@ -264,17 +264,13 @@ struct explicitly_convertible_to_string_like {
   }
 };
 
-TEST(FormatterTest, FormatExplicitlyConvertibleToStringLike) {
-  EXPECT_EQ("foo", fmt::format("{}", explicitly_convertible_to_string_like()));
-}
-
 std::ostream& operator<<(std::ostream& os,
                          explicitly_convertible_to_string_like) {
   return os << "bar";
 }
 
-TEST(FormatterTest, FormatExplicitlyConvertibleToStringLikeIgnoreInserter) {
-  EXPECT_EQ("foo", fmt::format("{}", explicitly_convertible_to_string_like()));
+TEST(FormatterTest, FormatExplicitlyConvertibleToStringLike) {
+  EXPECT_EQ("bar", fmt::format("{}", explicitly_convertible_to_string_like()));
 }
 
 #ifdef FMT_USE_STRING_VIEW
@@ -284,17 +280,13 @@ struct explicitly_convertible_to_std_string_view {
   }
 };
 
-TEST(FormatterTest, FormatExplicitlyConvertibleToStdStringView) {
-  EXPECT_EQ("foo", fmt::format("{}", explicitly_convertible_to_string_like()));
-}
-
 std::ostream& operator<<(std::ostream& os,
                          explicitly_convertible_to_std_string_view) {
   return os << "bar";
 }
 
-TEST(FormatterTest, FormatExplicitlyConvertibleToStdStringViewIgnoreInserter) {
-  EXPECT_EQ("foo",
-            fmt::format("{}", explicitly_convertible_to_std_string_view()));
+TEST(FormatterTest, FormatExplicitlyConvertibleToStdStringView) {
+  EXPECT_EQ("bar", fmt::format("{}", explicitly_convertible_to_string_like()));
 }
+
 #endif  // FMT_USE_STRING_VIEW