Fix #5114 - Avoid same-symbol qualifier-difference case falling into DtoCastClass

gen/classes.cpp

@@ -277,6 +277,12 @@ DValue *DtoCastClass(Loc loc, DValue *val, Type *_to) {
   Type *from = val->type->toBasetype();
   TypeClass *fc = static_cast<TypeClass *>(from);
 
+  // Qualifier-only casts are semantic no-ops; avoid dynamic cast routing.
+  if (dmd::equivalent(from, to)) {
+    Logger::println("qualifier-only cast");
+    return new DImValue(_to, DtoRVal(val));
+  }
+
   // copy DMD logic:
   // if to isBaseOf from with offset:   (to ? to + offset : null)
   // else if from is C++ and to is C++:  to
@@ -335,7 +341,7 @@ bool DtoIsObjcLinkage(Type *_to) {
     DtoResolveClass(to->sym);
     return to->sym->classKind == ClassKind::objc;
   }
-  
+
   return false;
 }
 
@@ -391,7 +397,7 @@ DValue *DtoDynamicCastInterface(Loc loc, DValue *val, Type *_to) {
   // In this case we want to call the Objective-C runtime to first
   // get a Class object from the `id`.
   // Then check if class_conformsToProtocol returns true,
-  // if it does, then we can cast and return the casted value, 
+  // if it does, then we can cast and return the casted value,
   // otherwise return null.
   if (DtoIsObjcLinkage(_to)) {
     llvm::Function *getClassFunc =
@@ -400,10 +406,10 @@ DValue *DtoDynamicCastInterface(Loc loc, DValue *val, Type *_to) {
     llvm::Function *kindOfProtocolFunc =
       getRuntimeFunction(loc, gIR->module, "class_conformsToProtocol");
 
-    // id -> Class 
+    // id -> Class
     LLValue *obj = DtoRVal(val);
     LLValue *objClass = gIR->CreateCallOrInvoke(getClassFunc, obj);
-    
+
     // Get prototype_t handle
     LLValue *protoTy = getNullPtr();
     if (auto ifhndl = _to->isClassHandle()->isInterfaceDeclaration()) {
@@ -413,7 +419,7 @@ DValue *DtoDynamicCastInterface(Loc loc, DValue *val, Type *_to) {
     // Class && kindOfProtocolFunc(Class) ? id : null
     LLValue *ret = gIR->ir->CreateSelect(
       gIR->CreateCallOrInvoke(kindOfProtocolFunc, objClass, protoTy),
-      obj, 
+      obj,
       getNullPtr()
     );
     return new DImValue(_to, ret);

gen/tocall.cpp

@@ -727,14 +727,15 @@ class ImplicitArgumentsBuilder {
           //  class pointer
           Type *thistype = gIR->func()->decl->vthis->type;
           if (thistype != iface->type) {
-            DImValue *dthis = new DImValue(thistype, DtoLoad(DtoType(thistype),thisptrLval));
-            thisptrLval = DtoAllocaDump(DtoCastClass(loc, dthis, iface->type));
+            auto thisVal = DtoLoad(DtoType(thistype), thisptrLval);
+            DImValue dthis(thistype, thisVal);
+            thisptrLval = DtoAllocaDump(DtoCastClass(loc, &dthis, iface->type));
           }
         }
       }
       args.push_back(thisptrLval);
     } else if (thiscall && dfnval && dfnval->vthis) {
-      
+
       if (objccall && directcall) {
 
         // ... or a Objective-c direct call argument

tests/codegen/gh5114.d

@@ -0,0 +1,79 @@
+module tests.codegen.gh5114;
+
+// During interface contract context setup, a qualifier-only cast for the same
+// interface symbol (e.g., `const(I)` -> `I`) must be handled as a repaint/
+// bitcast and must not route through dynamic interface cast lowering.
+//
+// True dynamic interface casts (different interface symbols) are still valid
+// and are covered below.
+//
+// RUN: %ldc -c %s
+// RUN: %ldc -unittest -main -run %s
+
+extern (D):
+
+interface I {
+    void fn() const
+    out {
+        // Positive case: contract body performs a true interface -> interface cast.
+        auto b = cast(const(B)) this;
+        assert(b !is null);
+        assert(b.b() == 22);
+    };
+}
+
+interface A {
+    int a();
+}
+
+interface B {
+    int b() const;
+}
+
+interface J : I {
+}
+
+class C : I, A, B {
+    override void fn() const
+    out (; true)
+    {
+    }
+
+    override int a() {
+        return 11;
+    }
+
+    override int b() const {
+        return 22;
+    }
+}
+
+class D : J, B {
+    override void fn() const
+    out (; true)
+    {
+    }
+
+    override int b() const {
+        return 22;
+    }
+}
+
+unittest {
+    A a = new C();
+
+    // True dynamic cast: interface -> interface, resolved via druntime cast hook.
+    B b = cast(B) a;
+
+    assert(b !is null);
+    assert(b.b() == 22);
+
+    // Ensure the interface contract executes in extern(D) call flow.
+    I i = cast(I) a;
+    i.fn();
+
+    // Derived-interface call flow for I's contract; this may route through a
+    // non-same-interface contract context conversion.
+    J j = new D();
+    j.fn();
+}