typescript-eslint · JoshuaKGoldberg · Aug 12, 2024 · Aug 6, 2024
diff --git a/packages/eslint-plugin/src/rules/no-unnecessary-type-parameters.ts b/packages/eslint-plugin/src/rules/no-unnecessary-type-parameters.ts
@@ -59,7 +59,13 @@ export default createRule({
 
           // Quick path: if the type parameter is used multiple times in the AST,
           // we don't need to dip into types to know it's repeated.
-          if (isTypeParameterRepeatedInAST(esTypeParameter, scope.references)) {
+          if (
+            isTypeParameterRepeatedInAST(
+              esTypeParameter,
+              scope.references,
+              node.body?.range[0] ?? node.returnType?.range[1],
+            )
+          ) {
             continue;
           }
 
@@ -86,18 +92,24 @@ export default createRule({
 function isTypeParameterRepeatedInAST(
   node: TSESTree.TSTypeParameter,
   references: Reference[],
+  startOfBody = Infinity,
 ): boolean {
   let total = 0;
 
   for (const reference of references) {
-    // References inside the type parameter's definition don't count.
+    // References inside the type parameter's definition don't count...
     if (
       reference.identifier.range[0] < node.range[1] &&
       reference.identifier.range[1] > node.range[0]
     ) {
       continue;
     }
 
+    // ...nor references that are outside the declaring signature.
+    if (reference.identifier.range[0] > startOfBody) {
+      continue;
+    }
+
     // Neither do references that aren't to the same type parameter,
     // namely value-land (non-type) identifiers of the type parameter's type,
     // and references to different type parameters or values.
@@ -110,7 +122,9 @@ function isTypeParameterRepeatedInAST(
 
     // If the type parameter is being used as a type argument, then we
     // know the type parameter is being reused and can't be reported.
-    if (reference.identifier.parent.type === AST_NODE_TYPES.TSTypeReference) {
+    if (
+      reference.identifier.parent.type === AST_NODE_TYPES.TSTypeReference
+    ) {
       const grandparent = skipConstituentsUpward(
         reference.identifier.parent.parent,
       );

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
@@ -445,6 +445,46 @@ ruleTester.run('no-unnecessary-type-parameters', rule, {
         { messageId: 'sole', data: { name: 'B' } },
       ],
     },
+    {
+      code: `
+        function foo<T>(_: T) {
+          const x: T = null!;
+          const y: T = null!;
+        }
+      `,
+      errors: [{ messageId: 'sole', data: { name: 'T' } }],
+    },
+    {
+      code: `
+        function foo<T>(_: T): void {
+          const x: T = null!;
+          const y: T = null!;
+        }
+      `,
+      errors: [{ messageId: 'sole', data: { name: 'T' } }],
+    },
+    {
+      code: `
+        function foo<T>(_: T): <T>(input: T) => T {
+          const x: T = null!;
+          const y: T = null!;
+        }
+      `,
+      errors: [{ messageId: 'sole', data: { name: 'T' } }],
+    },
+    {
+      code: `
+        function foo<T>(_: T) {
+          function withX(): T {
+            return null!;
+          }
+          function withY(): T {
+            return null!;
+          }
+        }
+      `,
+      errors: [{ messageId: 'sole', data: { name: 'T' } }],
+    },
     {
       code: `
         function parseYAML<T>(input: string): T {