Merge pull request #26669 from theblixguy/fix/SR-11298

[Sema] Setter has incorrect mutating-ness inside class-constrained protocol extension

Author: jrose-apple

lib/Sema/TypeCheckDecl.cpp

@@ -1964,8 +1964,17 @@ void TypeChecker::validateDecl(OperatorDecl *OD) {
 }
 
 bool swift::doesContextHaveValueSemantics(DeclContext *dc) {
-  if (Type contextTy = dc->getDeclaredInterfaceType())
-    return !contextTy->hasReferenceSemantics();
+  if (Type contextTy = dc->getDeclaredInterfaceType()) {
+    // If the decl context is an extension, then it could be imposing a class
+    // constraint (ex: where Self: SomeClass). Make sure we include that
+    // in our check as well.
+    auto extensionRequiresClass = false;
+    if (auto ED = dyn_cast<ExtensionDecl>(dc)) {
+      extensionRequiresClass =
+          ED->getGenericSignature()->requiresClass(ED->getSelfInterfaceType());
+    }
+    return !contextTy->hasReferenceSemantics() && !extensionRequiresClass;
+  }
   return false;
 }
 

test/decl/ext/extensions.swift

@@ -124,3 +124,23 @@ struct WrapperContext {
     static let propUsingMember = originalValue
   }
 }
+
+// SR-11298
+
+protocol SR_11298_P {}
+
+class SR_11298_C: SR_11298_P {
+  var property: String = ""
+}
+
+// Self: SR_11298_C requirement constrains this extension to SR_11298C and its subclasses.
+// Since this implies a class constraint, the setter should be implicitly nonmutating.
+extension SR_11298_P where Self: SR_11298_C {
+  var wrappingProperty: String {
+    get { return property }
+    set { property = newValue }
+  }
+}
+
+let instance = SR_11298_C()
+instance.wrappingProperty = "" // Okay