fix: forbidden override between generic function and concrete function (#3014)

Changqing-JING · web-flow · commit ffada5a3542e · 2026-04-25T13:07:26.000+08:00
diff --git a/src/diagnosticMessages.json b/src/diagnosticMessages.json
@@ -53,6 +53,7 @@
   "Definitive assignment has no effect on local variables.": 239,
   "Ambiguous operator overload '{0}' (conflicting overloads '{1}' and '{2}').": 240,
   "An interface or abstract method '{0}' cannot have type parameters.": 241,
+  "Cannot override generic method '{0}' with a non-generic method or vice versa.": 242,
 
   "Importing the table disables some indirect call optimizations.": 901,
   "Exporting the table disables some indirect call optimizations.": 902,
diff --git a/src/program.ts b/src/program.ts
@@ -1557,6 +1557,16 @@ export class Program extends DiagnosticEmitter {
     ) {
       let thisMethod = <FunctionPrototype>thisMember;
       let baseMethod = <FunctionPrototype>baseMember;
+      let thisIsGeneric = thisMethod.is(CommonFlags.Generic);
+      let baseIsGeneric = baseMethod.is(CommonFlags.Generic);
+      if (thisIsGeneric != baseIsGeneric) {
+        this.errorRelated(
+          DiagnosticCode.Cannot_override_generic_method_0_with_a_non_generic_method_or_vice_versa,
+          thisMethod.identifierNode.range, baseMethod.identifierNode.range,
+          thisMethod.name
+        );
+        return;
+      }
       if (!thisMethod.visibilityEquals(baseMethod)) {
         this.errorRelated(
           DiagnosticCode.Overload_signatures_must_all_be_public_private_or_protected,
diff --git a/src/resolver.ts b/src/resolver.ts
@@ -2933,11 +2933,24 @@ export class Resolver extends DiagnosticEmitter {
             incompatibleOverride = false;
           } else {
             if (baseMember.kind == ElementKind.FunctionPrototype) {
-              // Possibly generic. Resolve with same type arguments to obtain the correct one.
               let basePrototype = <FunctionPrototype>baseMember;
-              let baseFunction = this.resolveFunction(basePrototype, typeArguments, new Map(), ReportMode.Swallow);
-              if (baseFunction && instance.signature.isAssignableTo(baseFunction.signature, true)) {
-                incompatibleOverride = false;
+              let baseTypeParameterNodes = basePrototype.typeParameterNodes;
+              let baseIsGeneric = baseTypeParameterNodes != null && baseTypeParameterNodes.length > 0;
+              let instanceIsGeneric = typeArguments != null && typeArguments.length > 0;
+              if (baseIsGeneric != instanceIsGeneric) {
+                // Cannot mix generic and non-generic functions in an override chain
+                this.errorRelated(
+                  DiagnosticCode.Cannot_override_generic_method_0_with_a_non_generic_method_or_vice_versa,
+                  instance.identifierAndSignatureRange, baseMember.identifierAndSignatureRange,
+                  methodOrPropertyName
+                );
+                incompatibleOverride = false; // already reported
+              } else {
+                // Possibly generic. Resolve with same type arguments to obtain the correct one.
+                let baseFunction = this.resolveFunction(basePrototype, typeArguments, new Map(), ReportMode.Swallow);
+                if (baseFunction && instance.signature.isAssignableTo(baseFunction.signature, true)) {
+                  incompatibleOverride = false;
+                }
               }
             }
           }
@@ -3069,11 +3082,8 @@ export class Resolver extends DiagnosticEmitter {
             //   arguments to forward to the monomorphized child.
             // - generic child → generic base: OK; type args come from the base call site.
             // - non-generic child → non-generic base: OK; plain vtable override.
-            // FIXME: non-generic child → generic base is also mismatched (resolveFunction
-            //   would assert on typeArguments/typeParameterNodes length mismatch) but that
-            //   case is not yet guarded here. The correct fix is to replace this condition
-            //   with `boundFuncPrototype.is(Generic) == instance.is(Generic)`.
-            if (!boundFuncPrototype.is(CommonFlags.Generic) || instance.is(CommonFlags.Generic)) {
+            // - non-generic child → generic base: skip; mismatched generic-ness.
+            if (boundFuncPrototype.is(CommonFlags.Generic) == instance.is(CommonFlags.Generic)) {
               overrideInstance = this.resolveFunction(boundFuncPrototype, instance.typeArguments);
             }
           }
diff --git a/tests/compiler/override-generic-mismatch.json b/tests/compiler/override-generic-mismatch.json
@@ -0,0 +1,8 @@
+{
+  "asc_flags": [],
+  "stderr": [
+    "AS242: Cannot override generic method 'foo' with a non-generic method or vice versa.",
+    "AS242: Cannot override generic method 'bar' with a non-generic method or vice versa.",
+    "EOF"
+  ]
+}
diff --git a/tests/compiler/override-generic-mismatch.ts b/tests/compiler/override-generic-mismatch.ts
@@ -0,0 +1,25 @@
+// Non-generic method overriding generic method
+class A {
+  foo<T>(x: T): void {}
+}
+
+class B extends A {
+  foo(x: i32): void {}
+}
+
+let a:A = new B();
+a.foo<i32>(1);
+
+// Generic method overriding non-generic method
+class C {
+  bar(x: i32): void {}
+}
+
+class D extends C {
+  bar<T>(x: T): void {}
+}
+
+let c:C = new D();
+c.bar(1);
+
+ERROR("EOF");
