fix(eslint-plugin): [no-unnecessary-type-parameters] descend into all parts of mapped types in no-unnecessary-type-parameters (#9530)

Author: danvk

packages/eslint-plugin/src/rules/no-unnecessary-type-parameters.ts

@@ -271,10 +271,16 @@ function collectTypeParameterUsageCounts(
     // `isTypeReference` to avoid descending into all the properties of a
     // generic interface/class, e.g. `Map<K, V>`.
     else if (tsutils.isObjectType(type)) {
-      visitSymbolsListOnce(type.getProperties(), false);
+      const properties = type.getProperties();
+      visitSymbolsListOnce(properties, false);
 
       if (isMappedType(type)) {
         visitType(type.typeParameter, false);
+        if (properties.length === 0) {
+          // TS treats mapped types like `{[k in "a"]: T}` like `{a: T}`.
+          // They have properties, so we need to avoid double-counting.
+          visitType(type.templateType, false);
+        }
       }
 
       for (const typeArgument of type.aliasTypeArguments ?? []) {
@@ -372,6 +378,8 @@ function collectTypeParameterUsageCounts(
 
 interface MappedType extends ts.ObjectType {
   typeParameter?: ts.Type;
+  constraintType?: ts.Type;
+  templateType?: ts.Type;
 }
 
 function isMappedType(type: ts.Type): type is MappedType {

packages/eslint-plugin/tests/rules/no-unnecessary-type-parameters.test.ts

@@ -342,6 +342,12 @@ ruleTester.run('no-unnecessary-type-parameters', rule, {
         (token): token is Exclude<TSESTree.Token, Conditions & ExtractedToken> =>
           tokenType in conditions;
     `,
+    `
+      declare function mapObj<K extends string, V>(
+        obj: { [key in K]?: V },
+        fn: (key: K, val: V) => number,
+      ): number[];
+    `,
   ],
 
   invalid: [
@@ -608,5 +614,14 @@ ruleTester.run('no-unnecessary-type-parameters', rule, {
       code: 'type Fn = <T extends string>() => `a${T}b`;',
       errors: [{ messageId: 'sole', data: { name: 'T' } }],
     },
+    {
+      code: `
+        declare function mapObj<K extends string, V>(
+          obj: { [key in K]?: V },
+          fn: (key: K) => number,
+        ): number[];
+      `,
+      errors: [{ messageId: 'sole', data: { name: 'V' } }],
+    },
   ],
 });