Fix Issue 18958 - extern(C++) wchar, dchar mangling not correct

Author: TurkeyMan

src/dmd/cppmangle.d

@@ -1047,8 +1047,8 @@ public:
             case Tfloat80:               c = 'e';       break;
             case Tbool:                  c = 'b';       break;
             case Tchar:                  c = 'c';       break;
-            case Twchar:                 c = 't';       break;  // unsigned short (perhaps use 'Ds' ?
-            case Tdchar:                 c = 'w';       break;  // wchar_t (UTF-32) (perhaps use 'Di' ?
+            case Twchar:        p = 'D'; c = 's';       break;  // since C++11
+            case Tdchar:        p = 'D'; c = 'i';       break;  // since C++11
             case Timaginary32:  p = 'G'; c = 'f';       break;  // 'G' means imaginary
             case Timaginary64:  p = 'G'; c = 'd';       break;
             case Timaginary80:  p = 'G'; c = 'e';       break;

src/dmd/cppmanglewin.d

@@ -222,7 +222,10 @@ public:
             if (flags & IS_DMC)
                 buf.writestring("_Y"); // DigitalMars wchar_t
             else
-                buf.writestring("_W"); // Visual C++ wchar_t
+                buf.writestring("_S"); // Visual C++ char16_t (since C++11)
+            break;
+        case Tdchar:
+            buf.writestring("_U"); // Visual C++ char32_t (since C++11)
             break;
         default:
             visit(cast(Type)type);

test/compilable/cppmangle.d

@@ -731,3 +731,21 @@ extern(C++, Namespace18922)
 	else version(Windows)
 		static assert(func18922.mangleof == "?func18922@Namespace18922@@YAXUStruct18922@1@@Z");
 }
+
+/**************************************/
+// https://issues.dlang.org/show_bug.cgi?id=18958
+// Issue 18958 - extern(C++) wchar, dchar mangling not correct
+
+// TODO: update with wchar_t when mangling is supported...
+//extern (C++) void test_char_mangling(char, wchar, dchar, wchar_t);
+extern (C++) void test_char_mangling(char, wchar, dchar);
+version (Posix)
+{
+    static assert(test_char_mangling.mangleof == "_Z18test_char_manglingcDsDi");
+//    static assert(test_char_mangling.mangleof == "_Z18test_char_manglingcDsDiw");
+}
+version (Windows)
+{
+    static assert(test_char_mangling.mangleof == "?test_char_mangling@@YAXD_S_U@Z");
+//    static assert(test_char_mangling.mangleof == "?test_char_mangling@@YAXD_S_U_W@Z");
+}