fix Issue 16980 - wrong interface called

- ensure that class/interface size is finalized before
  determining the interface offsets

src/dclass.d

@@ -1876,9 +1876,15 @@ extern (C++) final class InterfaceDeclaration : ClassDeclaration
             //printf("\tX base %s\n", b.sym.toChars());
             if (this == b.sym)
             {
-                //printf("\tfound at offset %d\n", b.offset);
                 if (poffset)
-                    *poffset = b.offset;
+                {
+                    // 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;
+                }
+                // printf("\tfound at offset %d\n", b.offset);
                 return true;
             }
             if (isBaseOf(b, poffset))

test/runnable/test16980.d

@@ -0,0 +1,22 @@
+// https://issues.dlang.org/show_bug.cgi?id=16980
+interface A { void foo(); }
+interface B { void bar(); }
+interface AB : A, B {}
+class C : AB {
+    void foo() { assert(false, "Must never be called!"); }
+    void bar() {}
+}
+
+struct T() {
+    AB ab;
+    ~this() {
+        ab.bar(); // uses wrong vtable
+    }
+}
+
+T!() tinst; // triggers semantic3 of dtor from Module::semantic(1)
+
+void main()
+{
+    auto dst = T!()(new C);
+}