From c533ad157cfabb09a503074c4453b5a7d27920db Mon Sep 17 00:00:00 2001
From: Geod24 <pro.mathias.lang@gmail.com>
Date: Tue, 31 Mar 2020 12:36:30 +0900
Subject: [PATCH] Fix issue 20700 - Fwd refs leads to extern(C++, class|struct)
 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            |  7 +++++++
 src/dmd/attrib.h            |  1 +
 src/dmd/typesem.d           | 13 ++++++++-----
 test/compilable/cppmangle.d | 13 +++++++++++++
 4 files changed, 29 insertions(+), 5 deletions(-)

diff --git a/src/dmd/attrib.d b/src/dmd/attrib.d
index 7035ec3eba03..9c5978e8d059 100644
--- a/src/dmd/attrib.d
+++ b/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;
diff --git a/src/dmd/attrib.h b/src/dmd/attrib.h
index 40fa3940d5ad..f7e87b6105cd 100644
--- a/src/dmd/attrib.h
+++ b/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); }
 };
diff --git a/src/dmd/typesem.d b/src/dmd/typesem.d
index 98ee6c440693..714c74931abd 100644
--- a/src/dmd/typesem.d
+++ b/src/dmd/typesem.d
@@ -1810,13 +1810,16 @@ extern(C++) Type typeSemantic(Type t, const ref Loc loc, Scope* sc)
     Type visitStruct(TypeStruct mtype)
     {
         //printf("TypeStruct::semantic('%s')\n", mtype.toChars());
+        // If this is called via a forward reference,
+        // use the dsymbol's scope so that `extern(C++ class|struct)` applies
+        // https://issues.dlang.org/show_bug.cgi?id=20700
+        auto cppmangle = (mtype.sym && mtype.sym._scope) ?
+            mtype.sym._scope.cppmangle : sc.cppmangle;
+
         if (mtype.deco)
         {
-            if (sc && sc.cppmangle != CPPMANGLE.def)
-            {
-                if (mtype.cppmangle == CPPMANGLE.def)
-                    mtype.cppmangle = sc.cppmangle;
-            }
+            assert(cppmangle == CPPMANGLE.def ||
+                   cppmangle == mtype.cppmangle);
             return mtype;
         }
 
diff --git a/test/compilable/cppmangle.d b/test/compilable/cppmangle.d
index 1a84059a56e8..c2ce610cc920 100644
--- a/test/compilable/cppmangle.d
+++ b/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 Struct20700);
+    extern(C++, class) struct Struct20700 {}
+    void test20700_2(const ref Struct20700);
+
+    // Note: Needs to be `BV` (`class`), not `BU` (`struct`)
+    static assert(test20700_1.mangleof == `?test20700_1@@YAXABVStruct20700@@@Z`);
+    static assert(test20700_2.mangleof == `?test20700_2@@YAXABVStruct20700@@@Z`);
+}