fix Issue 17059 - incorrect circular reference with `is(Klass : Inter…

…face)`

- fixed by tweaking the fix for Issue 16980 (#6383)
- happened b/c TypeClass::implicitConv -> TypeClass::constConv checks
  for offset == 0, which triggered size finalization
- instead handle unfinalized classes in isBaseOf (new OFFSET_FWDREF)
  and only explicity finalize size before optimizing casts
- add a halt for any OFFSET_FWDREF leaking through to IR gen

src/aggregate.h

@@ -288,6 +288,7 @@ class ClassDeclaration : public AggregateDeclaration
     bool isBaseOf2(ClassDeclaration *cd);
 
     #define OFFSET_RUNTIME 0x76543210
+    #define OFFSET_FWDREF 0x76543211
     virtual bool isBaseOf(ClassDeclaration *cd, int *poffset);
 
     bool isBaseInfoComplete();

src/dclass.d

@@ -1027,6 +1027,7 @@ extern (C++) class ClassDeclaration : AggregateDeclaration
     }
 
     enum OFFSET_RUNTIME = 0x76543210;
+    enum OFFSET_FWDREF = 0x76543211;
 
     /*******************************************
      * Determine if 'this' is a base class of cd.
@@ -1878,11 +1879,8 @@ extern (C++) final class InterfaceDeclaration : ClassDeclaration
             {
                 if (poffset)
                 {
-                    // Need to determine correct offset if needed.
-                    // https://issues.dlang.org/show_bug.cgi?id=16980
-                    cd.size(loc);
-                    // HACK: using OFFSET_RUNTIME as error placeholder
-                    *poffset = cd.sizeok == SIZEOKdone ? b.offset : OFFSET_RUNTIME;
+                    // don't return incorrect offsets https://issues.dlang.org/show_bug.cgi?id=16980
+                    *poffset = cd.sizeok == SIZEOKdone ? b.offset : OFFSET_FWDREF;
                 }
                 // printf("\tfound at offset %d\n", b.offset);
                 return true;

src/e2ir.d

@@ -4034,7 +4034,11 @@ elem *toElem(Expression e, IRState *irs)
                      */
 
                     //printf("offset = %d\n", offset);
-                    if (offset)
+                    if (offset == ClassDeclaration.OFFSET_FWDREF)
+                    {
+                        assert(0, "unexpected forward reference");
+                    }
+                    else if (offset)
                     {
                         /* Rewrite cast as (e ? e + offset : null)
                          */

src/optimize.d

@@ -589,9 +589,16 @@ extern (C++) Expression Expression_optimize(Expression e, int result, bool keepL
             }
             if (e.type.ty == Tclass && e.e1.type.ty == Tclass)
             {
+                import ddmd.aggregate : SIZEOKdone;
+
                 // See if we can remove an unnecessary cast
                 ClassDeclaration cdfrom = e.e1.type.isClassHandle();
                 ClassDeclaration cdto = e.type.isClassHandle();
+                // Need to determine correct offset before optimizing away the cast.
+                // https://issues.dlang.org/show_bug.cgi?id=16980
+                cdfrom.size(e.loc);
+                assert(cdfrom.sizeok == SIZEOKdone);
+                assert(cdto.sizeok == SIZEOKdone || !cdto.isBaseOf(cdto, null));
                 int offset;
                 if (cdto.isBaseOf(cdfrom, &offset) && offset == 0)
                 {

test/compilable/test17059.d

@@ -0,0 +1,11 @@
+mixin template impl()
+{
+    alias T = typeof(this);
+    enum doImplement = is(T : I);
+
+    static if (doImplement)
+    {}
+}
+
+interface I {}
+class A : I {mixin impl;}