Merge pull request #16736 from DickvdBrink/issue/15208

Also check TypeAlias for unused type parameters

Author: sandersn

src/compiler/checker.ts

@@ -19470,6 +19470,9 @@ namespace ts {
                         case SyntaxKind.ConstructorType:
                             checkUnusedTypeParameters(<FunctionLikeDeclaration>node);
                             break;
+                        case SyntaxKind.TypeAliasDeclaration:
+                            checkUnusedTypeParameters(<TypeAliasDeclaration>node);
+                            break;
                     }
                 }
             }
@@ -19547,7 +19550,7 @@ namespace ts {
             }
         }
 
-        function checkUnusedTypeParameters(node: ClassDeclaration | ClassExpression | FunctionDeclaration | MethodDeclaration | FunctionExpression | ArrowFunction | ConstructorDeclaration | SignatureDeclaration | InterfaceDeclaration) {
+        function checkUnusedTypeParameters(node: ClassDeclaration | ClassExpression | FunctionDeclaration | MethodDeclaration | FunctionExpression | ArrowFunction | ConstructorDeclaration | SignatureDeclaration | InterfaceDeclaration | TypeAliasDeclaration) {
             if (compilerOptions.noUnusedLocals && !isInAmbientContext(node)) {
                 if (node.typeParameters) {
                     // Only report errors on the last declaration for the type parameter container;
@@ -21252,6 +21255,7 @@ namespace ts {
             checkTypeNameIsReserved(node.name, Diagnostics.Type_alias_name_cannot_be_0);
             checkTypeParameters(node.typeParameters);
             checkSourceElement(node.type);
+            registerForUnusedIdentifiersCheck(node);
         }
 
         function computeEnumMemberValues(node: EnumDeclaration) {

tests/baselines/reference/unusedTypeParameters10.errors.txt

@@ -0,0 +1,9 @@
+tests/cases/compiler/unusedTypeParameters10.ts(1,12): error TS6133: 'T' is declared but never used.
+
+
+==== tests/cases/compiler/unusedTypeParameters10.ts (1 errors) ====
+    type Alias<T> = { };
+               ~
+!!! error TS6133: 'T' is declared but never used.
+    type Alias2<T> = { x: T };
+    

tests/baselines/reference/unusedTypeParameters10.js

@@ -0,0 +1,6 @@
+//// [unusedTypeParameters10.ts]
+type Alias<T> = { };
+type Alias2<T> = { x: T };
+
+
+//// [unusedTypeParameters10.js]

tests/cases/compiler/unusedTypeParameters10.ts

@@ -0,0 +1,5 @@
+//@noUnusedLocals:true
+//@noUnusedParameters:true
+
+type Alias<T> = { };
+type Alias2<T> = { x: T };