Add error for failing to mark an mixin class as abstract

Author: rbuckton

src/compiler/checker.ts

@@ -34616,8 +34616,16 @@ namespace ts {
                         checkTypeAssignableTo(staticType, getTypeWithoutSignatures(staticBaseType), node.name || node,
                             Diagnostics.Class_static_side_0_incorrectly_extends_base_class_static_side_1);
                     }
-                    if (baseConstructorType.flags & TypeFlags.TypeVariable && !isMixinConstructorType(staticType)) {
-                        error(node.name || node, Diagnostics.A_mixin_class_must_have_a_constructor_with_a_single_rest_parameter_of_type_any);
+                    if (baseConstructorType.flags & TypeFlags.TypeVariable) {
+                        if (!isMixinConstructorType(staticType)) {
+                            error(node.name || node, Diagnostics.A_mixin_class_must_have_a_constructor_with_a_single_rest_parameter_of_type_any);
+                        }
+                        else {
+                            const constructSignatures = getSignaturesOfType(baseConstructorType, SignatureKind.Construct);
+                            if (constructSignatures.some(signature => signature.flags & SignatureFlags.Abstract) && !hasSyntacticModifier(node, ModifierFlags.Abstract)) {
+                                error(node.name || node, Diagnostics.A_mixin_class_that_extends_from_a_type_variable_containing_an_abstract_construct_signature_must_also_be_declared_abstract);
+                            }
+                        }
                     }
 
                     if (!(staticBaseType.symbol && staticBaseType.symbol.flags & SymbolFlags.Class) && !(baseConstructorType.flags & TypeFlags.TypeVariable)) {

src/compiler/diagnosticMessages.json

@@ -3028,6 +3028,10 @@
         "category": "Error",
         "code": 2792
     },
+    "A mixin class that extends from a type variable containing an abstract construct signature must also be declared 'abstract'.": {
+        "category": "Error",
+        "code": 2793
+    },
 
     "Import declaration '{0}' is using private name '{1}'.": {
         "category": "Error",

tests/baselines/reference/mixinAbstractClasses.2.errors.txt

@@ -1,14 +1,18 @@
-tests/cases/conformance/classes/mixinAbstractClasses.2.ts(20,7): error TS2515: Non-abstract class 'DerivedFromAbstract' does not implement inherited abstract member 'abstractBaseMethod' from class 'AbstractBase & Mixin'.
-tests/cases/conformance/classes/mixinAbstractClasses.2.ts(24,1): error TS2511: Cannot create an instance of an abstract class.
+tests/cases/conformance/classes/mixinAbstractClasses.2.ts(7,11): error TS2793: A mixin class that extends from a type variable containing an abstract construct signature must also be declared 'abstract'.
+tests/cases/conformance/classes/mixinAbstractClasses.2.ts(21,7): error TS2515: Non-abstract class 'DerivedFromAbstract' does not implement inherited abstract member 'abstractBaseMethod' from class 'AbstractBase & Mixin'.
+tests/cases/conformance/classes/mixinAbstractClasses.2.ts(25,1): error TS2511: Cannot create an instance of an abstract class.
 
 
-==== tests/cases/conformance/classes/mixinAbstractClasses.2.ts (2 errors) ====
+==== tests/cases/conformance/classes/mixinAbstractClasses.2.ts (3 errors) ====
     interface Mixin {
         mixinMethod(): void;
     }
 
     function Mixin<TBaseClass extends abstract new (...args: any) => any>(baseClass: TBaseClass): TBaseClass & (abstract new (...args: any) => Mixin) {
-        abstract class MixinClass extends baseClass implements Mixin {
+        // error expected: A mixin class that extends from a type variable containing an abstract construct signature must also be declared 'abstract'.
+        class MixinClass extends baseClass implements Mixin {
+              ~~~~~~~~~~
+!!! error TS2793: A mixin class that extends from a type variable containing an abstract construct signature must also be declared 'abstract'.
             mixinMethod() {
             }
         }

tests/baselines/reference/mixinAbstractClasses.2.js

@@ -4,7 +4,8 @@ interface Mixin {
 }
 
 function Mixin<TBaseClass extends abstract new (...args: any) => any>(baseClass: TBaseClass): TBaseClass & (abstract new (...args: any) => Mixin) {
-    abstract class MixinClass extends baseClass implements Mixin {
+    // error expected: A mixin class that extends from a type variable containing an abstract construct signature must also be declared 'abstract'.
+    class MixinClass extends baseClass implements Mixin {
         mixinMethod() {
         }
     }
@@ -26,6 +27,7 @@ new MixedBase();
 
 //// [mixinAbstractClasses.2.js]
 function Mixin(baseClass) {
+    // error expected: A mixin class that extends from a type variable containing an abstract construct signature must also be declared 'abstract'.
     class MixinClass extends baseClass {
         mixinMethod() {
         }

tests/baselines/reference/mixinAbstractClasses.2.symbols

@@ -16,38 +16,39 @@ function Mixin<TBaseClass extends abstract new (...args: any) => any>(baseClass:
 >args : Symbol(args, Decl(mixinAbstractClasses.2.ts, 4, 122))
 >Mixin : Symbol(Mixin, Decl(mixinAbstractClasses.2.ts, 2, 1), Decl(mixinAbstractClasses.2.ts, 0, 0))
 
-    abstract class MixinClass extends baseClass implements Mixin {
+    // error expected: A mixin class that extends from a type variable containing an abstract construct signature must also be declared 'abstract'.
+    class MixinClass extends baseClass implements Mixin {
 >MixinClass : Symbol(MixinClass, Decl(mixinAbstractClasses.2.ts, 4, 147))
 >baseClass : Symbol(baseClass, Decl(mixinAbstractClasses.2.ts, 4, 70))
 >Mixin : Symbol(Mixin, Decl(mixinAbstractClasses.2.ts, 2, 1), Decl(mixinAbstractClasses.2.ts, 0, 0))
 
         mixinMethod() {
->mixinMethod : Symbol(MixinClass.mixinMethod, Decl(mixinAbstractClasses.2.ts, 5, 66))
+>mixinMethod : Symbol(MixinClass.mixinMethod, Decl(mixinAbstractClasses.2.ts, 6, 57))
         }
     }
     return MixinClass;
 >MixinClass : Symbol(MixinClass, Decl(mixinAbstractClasses.2.ts, 4, 147))
 }
 
 abstract class AbstractBase {
->AbstractBase : Symbol(AbstractBase, Decl(mixinAbstractClasses.2.ts, 10, 1))
+>AbstractBase : Symbol(AbstractBase, Decl(mixinAbstractClasses.2.ts, 11, 1))
 
     abstract abstractBaseMethod(): void;
->abstractBaseMethod : Symbol(AbstractBase.abstractBaseMethod, Decl(mixinAbstractClasses.2.ts, 12, 29))
+>abstractBaseMethod : Symbol(AbstractBase.abstractBaseMethod, Decl(mixinAbstractClasses.2.ts, 13, 29))
 }
 
 const MixedBase = Mixin(AbstractBase);
->MixedBase : Symbol(MixedBase, Decl(mixinAbstractClasses.2.ts, 16, 5))
+>MixedBase : Symbol(MixedBase, Decl(mixinAbstractClasses.2.ts, 17, 5))
 >Mixin : Symbol(Mixin, Decl(mixinAbstractClasses.2.ts, 2, 1), Decl(mixinAbstractClasses.2.ts, 0, 0))
->AbstractBase : Symbol(AbstractBase, Decl(mixinAbstractClasses.2.ts, 10, 1))
+>AbstractBase : Symbol(AbstractBase, Decl(mixinAbstractClasses.2.ts, 11, 1))
 
 // error expected: Non-abstract class 'DerivedFromAbstract' does not implement inherited abstract member 'abstractBaseMethod' from class 'AbstractBase & Mixin'.
 class DerivedFromAbstract extends MixedBase {
->DerivedFromAbstract : Symbol(DerivedFromAbstract, Decl(mixinAbstractClasses.2.ts, 16, 38))
->MixedBase : Symbol(MixedBase, Decl(mixinAbstractClasses.2.ts, 16, 5))
+>DerivedFromAbstract : Symbol(DerivedFromAbstract, Decl(mixinAbstractClasses.2.ts, 17, 38))
+>MixedBase : Symbol(MixedBase, Decl(mixinAbstractClasses.2.ts, 17, 5))
 }
 
 // error expected: Cannot create an instance of an abstract class.
 new MixedBase();
->MixedBase : Symbol(MixedBase, Decl(mixinAbstractClasses.2.ts, 16, 5))
+>MixedBase : Symbol(MixedBase, Decl(mixinAbstractClasses.2.ts, 17, 5))
 

tests/baselines/reference/mixinAbstractClasses.2.types

@@ -10,7 +10,8 @@ function Mixin<TBaseClass extends abstract new (...args: any) => any>(baseClass:
 >baseClass : TBaseClass
 >args : any
 
-    abstract class MixinClass extends baseClass implements Mixin {
+    // error expected: A mixin class that extends from a type variable containing an abstract construct signature must also be declared 'abstract'.
+    class MixinClass extends baseClass implements Mixin {
 >MixinClass : MixinClass
 >baseClass : TBaseClass
 

tests/cases/conformance/classes/mixinAbstractClasses.2.ts

@@ -6,7 +6,8 @@ interface Mixin {
 }
 
 function Mixin<TBaseClass extends abstract new (...args: any) => any>(baseClass: TBaseClass): TBaseClass & (abstract new (...args: any) => Mixin) {
-    abstract class MixinClass extends baseClass implements Mixin {
+    // error expected: A mixin class that extends from a type variable containing an abstract construct signature must also be declared 'abstract'.
+    class MixinClass extends baseClass implements Mixin {
         mixinMethod() {
         }
     }