Fix issue 20700 - Forward references leads to extern(C++, class|struc…

…t) being ignored

On the long run, it looks like we should just move the setScope to AttribDeclaration,
but this is a simple bugfix to target stable.

src/dmd/attrib.d

@@ -435,6 +435,13 @@ extern (C++) final class CPPMangleDeclaration : AttribDeclaration
             sc.aligndecl, sc.inlining);
     }
 
+    override void setScope(Scope* sc)
+    {
+        if (decl)
+            Dsymbol.setScope(sc); // for forward reference
+        return AttribDeclaration.setScope(sc);
+    }
+
     override const(char)* toChars() const
     {
         return toString().ptr;

src/dmd/attrib.h

@@ -88,6 +88,7 @@ class CPPMangleDeclaration : public AttribDeclaration
 
     Dsymbol *syntaxCopy(Dsymbol *s);
     Scope *newScope(Scope *sc);
+    void setScope(Scope *sc);
     const char *toChars() const;
     void accept(Visitor *v) { v->visit(this); }
 };

test/compilable/cppmangle.d

@@ -1251,3 +1251,16 @@ extern(C++, `bar`)
 {
     void func19542(T)();
 }
+
+// https://issues.dlang.org/show_bug.cgi?id=20700
+version (Windows)
+extern(C++)
+{
+    void test20700_1(const ref Struct200700);
+    extern(C++, class) struct Struct200700 {}
+    void test20700_2(const ref Struct200700);
+
+    // Note: Needs to be `BV` (`class`), not `BU` (`struct`)
+    static assert(test20700_1.mangleof == `?test20700_1@@YAXABVStruct200700@@@Z`);
+    static assert(test20700_2.mangleof == `?test20700_2@@YAXABVStruct200700@@@Z`);
+}