From 73bed45b7ab4ffeffbf463c08042c97c3094da3c Mon Sep 17 00:00:00 2001
From: Victor Zverovich <victor.zverovich@gmail.com>
Date: Wed, 18 Jul 2018 14:12:10 -0700
Subject: [PATCH] Add support for types explicitly convertible to
 fmt::string_view

---
 include/fmt/core.h  |  9 ++++++++-
 test/format-test.cc | 17 ++++++++++++++---
 2 files changed, 22 insertions(+), 4 deletions(-)

diff --git a/include/fmt/core.h b/include/fmt/core.h
index f35bfc3a..c65709ac 100644
--- a/include/fmt/core.h
+++ b/include/fmt/core.h
@@ -672,10 +672,17 @@ inline typename std::enable_if<
     typed_value<C, int_type>>::type
   make_value(const T &val) { return static_cast<int>(val); }
 
+template <typename C, typename T, typename Char = typename C::char_type>
+inline typename std::enable_if<
+    std::is_constructible<basic_string_view<Char>, T>::value,
+    typed_value<C, string_type>>::type
+  make_value(const T &val) { return string_view(val); }
+
 template <typename C, typename T, typename Char = typename C::char_type>
 inline typename std::enable_if<
     !convert_to_int<T, Char>::value &&
-    !std::is_convertible<T, basic_string_view<Char>>::value,
+    !std::is_convertible<T, basic_string_view<Char>>::value &&
+    !std::is_constructible<string_view, T>::value,
     // Implicit conversion to std::string is not handled here because it's
     // unsafe: https://github.com/fmtlib/fmt/issues/729
     typed_value<C, custom_type>>::type
diff --git a/test/format-test.cc b/test/format-test.cc
index d283d913..2b3ac5fa 100644
--- a/test/format-test.cc
+++ b/test/format-test.cc
@@ -1085,14 +1085,25 @@ TEST(FormatterTest, FormatStdStringView) {
 }
 #endif
 
-struct ConvertibleToStringView {
+struct implicitly_convertible_to_string_view {
   operator fmt::string_view() const { return "foo"; }
 };
 
-TEST(FormatterTest, FormatConvertibleToStringView) {
-  EXPECT_EQ("foo", format("{}", ConvertibleToStringView()));
+TEST(FormatterTest, FormatImplicitlyConvertibleToStringView) {
+  EXPECT_EQ("foo", format("{}", implicitly_convertible_to_string_view()));
 }
 
+// std::is_constructible is broken in MSVC until version 2015.
+#if !FMT_MSC_VER || FMT_MSC_VER >= 1900
+struct explicitly_convertible_to_string_view {
+  explicit operator fmt::string_view() const { return "foo"; }
+};
+
+TEST(FormatterTest, FormatExplicitlyConvertibleToStringView) {
+  EXPECT_EQ("foo", format("{}", explicitly_convertible_to_string_view()));
+}
+#endif
+
 FMT_BEGIN_NAMESPACE
 template <>
 struct formatter<Date> {