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

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)