Disallow formatting of wide strings when using a narrow string formatter.

format.h

@@ -361,7 +361,10 @@ class CharTraits<char> : public BasicCharTraits<char> {
   // Conversion from wchar_t to char is not allowed.
   static char ConvertChar(wchar_t);
 
- public:
+  // Conversion from const wchar_t * to const char * is not allowed.
+  static const wchar_t *check(const wchar_t *s);
+
+public:
   typedef const wchar_t *UnsupportedStrType;
 
   static char ConvertChar(char value) { return value; }
@@ -386,6 +389,8 @@ class CharTraits<wchar_t> : public BasicCharTraits<wchar_t> {
 
   static StringValue<wchar_t> convert(StringValue<wchar_t> s) { return s; }
 
+  static const wchar_t *check(const wchar_t *s) { return s; }
+
   template <typename T>
   static int FormatFloat(wchar_t *buffer, std::size_t size,
       const wchar_t *format, unsigned width, int precision, T value);
@@ -636,7 +641,7 @@ class MakeArg : public Arg {
 
   void SetString(WStringRef str) {
     type = WSTRING;
-    wstring.value = str.c_str();
+    wstring.value = CharTraits<Char>::check(str.c_str());
     wstring.size = str.size();
   }
 

test/CMakeLists.txt

@@ -35,6 +35,7 @@ expect_compile_error("const volatile char s[] = \"test\"; (fmt::internal::MakeAr
 
 # MakeArg<char> doesn't accept wchar_t.
 expect_compile_error("fmt::internal::MakeArg<char>(L'a');")
+expect_compile_error("fmt::internal::MakeArg<char>(L\"test\");")
 
 # Writing a wide character to a character stream Writer is forbidden.
 expect_compile_error("fmt::Writer() << L'a';")