fix(eslint-plugin): [no-unnecessary-type-parameters] check mapped constraint types if necessary (typescript-eslint#9740)

JoshuaKGoldberg · web-flow · commit da02f6118058 · 2024-08-19T07:18:28.000-04:00
diff --git a/packages/eslint-plugin/src/rules/no-unnecessary-type-parameters.ts b/packages/eslint-plugin/src/rules/no-unnecessary-type-parameters.ts
@@ -303,7 +303,7 @@ function collectTypeParameterUsageCounts(
         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);
+          visitType(type.templateType ?? type.constraintType, false);
         }
       }
 
diff --git a/packages/eslint-plugin/tests/rules/no-unnecessary-type-parameters.test.ts b/packages/eslint-plugin/tests/rules/no-unnecessary-type-parameters.test.ts
@@ -348,6 +348,33 @@ ruleTester.run('no-unnecessary-type-parameters', rule, {
         fn: (key: K, val: V) => number,
       ): number[];
     `,
+    `
+      declare function mappedReturnType<T extends string>(
+        x: T,
+      ): { [K in T]: Capitalize<K> };
+
+      function inferredMappedReturnType<T extends string>(x: T) {
+        return mappedReturnType(x);
+      }
+    `,
+    `
+      declare function mappedReturnType<T extends string>(
+        x: T,
+      ): { [K in T]: Capitalize<K> };
+
+      function inferredMappedReturnType<T extends string>(x: T) {
+        return () => mappedReturnType(x);
+      }
+    `,
+    `
+      declare function mappedReturnType<T extends string>(
+        x: T,
+      ): { [K in T]: Capitalize<K> };
+
+      function inferredMappedReturnType<T extends string>(x: T) {
+        return [{ value: () => mappedReturnType(x) }];
+      }
+    `,
   ],
 
   invalid: [

Original file line number	Diff line number	Diff line change
`@@ -303,7 +303,7 @@ function collectTypeParameterUsageCounts(`
`303`	`303`	`if (properties.length === 0) {`
`304`	`304`	// TS treats mapped types like `{[k in "a"]: T}` like `{a: T}`.
`305`	`305`	`// They have properties, so we need to avoid double-counting.`
`306`		`- visitType(type.templateType, false);`
	`306`	`+ visitType(type.templateType ?? type.constraintType, false);`
`307`	`307`	`}`
`308`	`308`	`}`
`309`	`309`