Disable unsafe implicit conversion to std::string (#729)

include/fmt/core.h

@@ -629,8 +629,9 @@ inline typename std::enable_if<
 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, std::basic_string<Char>>::value,
+    !std::is_convertible<T, basic_string_view<Char>>::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
   make_value(const T &val) { return val; }
 

test/compile-test/CMakeLists.txt

@@ -9,8 +9,7 @@ set(CMAKE_REQUIRED_FLAGS ${CPP14_FLAG})
 function (generate_source result fragment)
   set(${result} "
   #define FMT_HEADER_ONLY 1
-  #include \"fmt/posix.h\"
-  #include \"fmt/ostream.h\"
+  #include \"fmt/format.h\"
   int main() {
     ${fragment}
   }
@@ -58,6 +57,14 @@ expect_compile_error("fmt::format(\"{}\", L\"foo\");")
 # mixing UTF-8 with UTF-16/32 can result in an invalid output.
 expect_compile_error("fmt::format(L\"{}\", \"foo\");")
 
+# Formatting a wide string with a narrow format string is forbidden.
+expect_compile_error("
+  struct S {
+    operator std::string() const { return std::string(); }
+  };
+  fmt::format(\"{}\", S());
+")
+
 # Make sure that compiler features detected in the header
 # match the features detected in CMake.
 if (SUPPORTS_USER_DEFINED_LITERALS)

test/format-test.cc

@@ -1072,16 +1072,6 @@ TEST(FormatterTest, FormatStdStringView) {
 }
 #endif
 
-struct ConvertibleToString {
-  std::string s;
-  ConvertibleToString() : s("foo") {}
-  operator const std::string &() const { return s; }
-};
-
-TEST(FormatterTest, FormatConvertibleToString) {
-  EXPECT_EQ("foo", format("{}", ConvertibleToString()));
-}
-
 struct ConvertibleToStringView {
   operator fmt::string_view() const { return "foo"; }
 };

test/util-test.cc

@@ -402,7 +402,7 @@ TEST(UtilTest, BitCast) {
     uint32_t u[2];
   };
   auto s = fmt::internal::bit_cast<S>(uint64_t(42));
-  EXPECT_EQ(fmt::internal::bit_cast<uint64_t>(s), 42);
+  EXPECT_EQ(fmt::internal::bit_cast<uint64_t>(s), 42u);
   s = fmt::internal::bit_cast<S>(uint64_t(~0ull));
   EXPECT_EQ(fmt::internal::bit_cast<uint64_t>(s), ~0ull);
 }