From 21429c864648341b7ef7d7c20e96df84ba16bae2 Mon Sep 17 00:00:00 2001
From: Victor Zverovich <victor.zverovich@gmail.com>
Date: Sun, 21 Jan 2018 18:53:38 -0800
Subject: [PATCH] Revert "Replace a bunch of craft with type_traits"

This reverts commit e79588d6c1e654cb2833d496042b14807c8cd818.
---
 include/fmt/core.h    | 40 +++++++++++++++++++++++++++++++++++++---
 include/fmt/ostream.h | 18 ++++++++++++++----
 test/util-test.cc     |  6 +++---
 3 files changed, 54 insertions(+), 10 deletions(-)

diff --git a/include/fmt/core.h b/include/fmt/core.h
index 024a7912..a73fa829 100644
--- a/include/fmt/core.h
+++ b/include/fmt/core.h
@@ -338,13 +338,47 @@ inline void require_wchar() {
       "formatting of wide characters into a narrow output is disallowed");
 }
 
-template <typename T, bool ENABLE = true>
-struct convert_to_int {
+using yes = char[1];
+using no = char[2];
+
+yes &convert(unsigned long long);
+no &convert(...);
+
+template<typename T, bool ENABLE_CONVERSION>
+struct convert_to_int_impl {
+  enum { value = ENABLE_CONVERSION };
+};
+
+template<typename T, bool ENABLE_CONVERSION>
+struct convert_to_int_impl2 {
+  enum { value = false };
+};
+
+template<typename T>
+struct convert_to_int_impl2<T, true> {
   enum {
-    value = !std::is_arithmetic<T>::value && std::is_convertible<T, int>::value
+    // Don't convert arithmetic types.
+    value = convert_to_int_impl<T, !std::is_arithmetic<T>::value>::value
   };
 };
 
+template<typename T>
+struct convert_to_int {
+  enum {
+    enable_conversion = sizeof(convert(std::declval<T>())) == sizeof(yes)
+  };
+  enum { value = convert_to_int_impl2<T, enable_conversion>::value };
+};
+
+#define FMT_DISABLE_CONVERSION_TO_INT(Type) \
+  template <> \
+  struct convert_to_int<Type> { enum { value = 0 }; }
+
+// Silence warnings about convering float to int.
+FMT_DISABLE_CONVERSION_TO_INT(float);
+FMT_DISABLE_CONVERSION_TO_INT(double);
+FMT_DISABLE_CONVERSION_TO_INT(long double);
+
 template <typename Char>
 struct named_arg_base;
 
diff --git a/include/fmt/ostream.h b/include/fmt/ostream.h
index 6e26817f..09b85608 100644
--- a/include/fmt/ostream.h
+++ b/include/fmt/ostream.h
@@ -47,12 +47,22 @@ class FormatBuf : public std::basic_streambuf<Char> {
   }
 };
 
-template <typename T>
-struct convert_to_int<T,
-    sizeof(std::declval<std::ostream>() << std::declval<T>()) != 0> {
+yes &convert(std::ostream &);
+
+struct DummyStream : std::ostream {
+  DummyStream();  // Suppress a bogus warning in MSVC.
+  // Hide all operator<< overloads from std::ostream.
+  void operator<<(null<>);
+};
+
+no &operator<<(std::ostream &, int);
+
+template<typename T>
+struct convert_to_int_impl<T, true> {
   // Convert to int only if T doesn't have an overloaded operator<<.
   enum {
-    value = false
+    value = sizeof(convert(std::declval<DummyStream&>() << std::declval<T>()))
+            == sizeof(no)
   };
 };
 
diff --git a/test/util-test.cc b/test/util-test.cc
index 4f4f04c7..65d8e5f8 100644
--- a/test/util-test.cc
+++ b/test/util-test.cc
@@ -827,15 +827,15 @@ TEST(UtilTest, ReportWindowsError) {
 enum TestEnum2 {};
 
 TEST(UtilTest, ConvertToInt) {
-  EXPECT_FALSE(fmt::internal::convert_to_int<char>::value);
-  EXPECT_FALSE(fmt::internal::convert_to_int<const char *>::value);
+  EXPECT_TRUE(fmt::internal::convert_to_int<char>::enable_conversion);
+  EXPECT_FALSE(fmt::internal::convert_to_int<const char *>::enable_conversion);
   EXPECT_TRUE(fmt::internal::convert_to_int<TestEnum2>::value);
 }
 
 #if FMT_USE_ENUM_BASE
 enum TestEnum : char {TestValue};
 TEST(UtilTest, IsEnumConvertibleToInt) {
-  EXPECT_TRUE(fmt::internal::convert_to_int<TestEnum>::value);
+  EXPECT_TRUE(fmt::internal::convert_to_int<TestEnum>::enable_conversion);
 }
 #endif