typescript-eslint · kirkwaiblinger · Apr 28, 2025 · Mar 2, 2025 · Mar 2, 2025 · Mar 8, 2025
diff --git a/packages/eslint-plugin/src/rules/no-unnecessary-type-parameters.ts b/packages/eslint-plugin/src/rules/no-unnecessary-type-parameters.ts
@@ -10,6 +10,7 @@ import type { MakeRequired } from '../util';
 import {
   createRule,
   getParserServices,
+  getWrappingFixer,
   nullThrows,
   NullThrowsReasons,
 } from '../util';
@@ -108,7 +109,28 @@ export default createRule({
                 for (const reference of smTypeParameterVariable.references) {
                   if (reference.isTypeReference) {
                     const referenceNode = reference.identifier;
-                    yield fixer.replaceText(referenceNode, constraintText);
+                    const isComplexType =
+                      constraint?.type === AST_NODE_TYPES.TSUnionType ||
+                      constraint?.type === AST_NODE_TYPES.TSIntersectionType ||
+                      constraint?.type === AST_NODE_TYPES.TSConditionalType;
+                    const hasMatchingAncestorType = [
+                      AST_NODE_TYPES.TSArrayType,
+                      AST_NODE_TYPES.TSIndexedAccessType,
+                      AST_NODE_TYPES.TSIntersectionType,
+                      AST_NODE_TYPES.TSUnionType,
+                      // eslint-disable-next-line @typescript-eslint/no-non-null-assertion
+                    ].some(type => referenceNode.parent.parent!.type === type);
+                    if (isComplexType && hasMatchingAncestorType) {
+                      const fixResult = getWrappingFixer({
+                        node: referenceNode,
+                        innerNode: constraint,
+                        sourceCode: context.sourceCode,
+                        wrap: constraintNode => constraintNode,
+                      })(fixer);
+                      yield fixResult;
+                    } else {
+                      yield fixer.replaceText(referenceNode, constraintText);
+                    }
                   }
                 }
 

diff --git a/packages/eslint-plugin/src/util/getWrappingFixer.ts b/packages/eslint-plugin/src/util/getWrappingFixer.ts
@@ -32,7 +32,7 @@ interface WrappingFixerParams {
  */
 export function getWrappingFixer(
   params: WrappingFixerParams,
-): TSESLint.ReportFixFunction {
+): (fixer: TSESLint.RuleFixer) => TSESLint.RuleFix {
   const { node, innerNode = node, sourceCode, wrap } = params;
   const innerNodes = Array.isArray(innerNode) ? innerNode : [innerNode];
 

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
@@ -1744,5 +1744,172 @@ class Joiner {
         },
       ],
     },
+    {
+      code: `
+function join<T extends string | number>(els: T[]) {
+  return els.map(el => '' + el).join(',');
+}
+      `,
+      errors: [
+        {
+          data: { descriptor: 'function', name: 'T', uses: 'used only once' },
+          messageId: 'sole',
+          suggestions: [
+            {
+              messageId: 'replaceUsagesWithConstraint',
+              output: `
+function join(els: (string | number)[]) {
+  return els.map(el => '' + el).join(',');
+}
+      `,
+            },
+          ],
+        },
+      ],
+    },
+    {
+      code: `
+function join<T extends string & number>(els: T[]) {
+  return els.map(el => '' + el).join(',');
+}
+      `,
+      errors: [
+        {
+          data: { descriptor: 'function', name: 'T', uses: 'used only once' },
+          messageId: 'sole',
+          suggestions: [
+            {
+              messageId: 'replaceUsagesWithConstraint',
+              output: `
+function join(els: (string & number)[]) {
+  return els.map(el => '' + el).join(',');
+}
+      `,
+            },
+          ],
+        },
+      ],
+    },
+    {
+      code: `
+function join<T extends (string & number) | boolean>(els: T[]) {
+  return els.map(el => '' + el).join(',');
+}
+      `,
+      errors: [
+        {
+          data: { descriptor: 'function', name: 'T', uses: 'used only once' },
+          messageId: 'sole',
+          suggestions: [
+            {
+              messageId: 'replaceUsagesWithConstraint',
+              output: `
+function join(els: ((string & number) | boolean)[]) {
+  return els.map(el => '' + el).join(',');
+}
+      `,
+            },
+          ],
+        },
+      ],
+    },
+    {
+      code: noFormat`
+function join<T extends (string | number)>(els: T[]) {
+  return els.map(el => '' + el).join(',');
+}
+      `,
+      errors: [
+        {
+          data: { descriptor: 'function', name: 'T', uses: 'used only once' },
+          messageId: 'sole',
+          suggestions: [
+            {
+              messageId: 'replaceUsagesWithConstraint',
+              output: `
+function join(els: (string | number)[]) {
+  return els.map(el => '' + el).join(',');
+}
+      `,
+            },
+          ],
+        },
+      ],
+    },
+    {
+      code: `
+function join<T extends { hoge: string } | { hoge: number }>(els: T['hoge'][]) {
+  return els.map(el => '' + el).join(',');
+}
+      `,
+      errors: [
+        {
+          data: { descriptor: 'function', name: 'T', uses: 'used only once' },
+          messageId: 'sole',
+          suggestions: [
+            {
+              messageId: 'replaceUsagesWithConstraint',
+              output: `
+function join(els: ({ hoge: string } | { hoge: number })['hoge'][]) {
+  return els.map(el => '' + el).join(',');
+}
+      `,
+            },
+          ],
+        },
+      ],
+    },
+    {
+      code: `
+type A = string;
+type B = string;
+type C = string;
+declare function f<T extends A | B>(): T & C;
+      `,
+      errors: [
+        {
+          data: { descriptor: 'function', name: 'T', uses: 'used only once' },
+          messageId: 'sole',
+          suggestions: [
+            {
+              messageId: 'replaceUsagesWithConstraint',
+              output: `
+type A = string;
+type B = string;
+type C = string;
+declare function f(): (A | B) & C;
+      `,
+            },
+          ],
+        },
+      ],
+    },
+    {
+      code: `
+type A = string;
+type B = string;
+type C = string;
+type D = string;
+declare function f<T extends A extends B ? C : D>(): T | null;
+      `,
+      errors: [
+        {
+          data: { descriptor: 'function', name: 'T', uses: 'used only once' },
+          messageId: 'sole',
+          suggestions: [
+            {
+              messageId: 'replaceUsagesWithConstraint',
+              output: `
+type A = string;
+type B = string;
+type C = string;
+type D = string;
+declare function f(): (A extends B ? C : D) | null;
+      `,
+            },
+          ],
+        },
+      ],
+    },
   ],
 });