From 681ac8ed2df7afa44f48790a91f6d57030814505 Mon Sep 17 00:00:00 2001
From: Hasegawa-Yukihiro <y.h.baskeeee@icloud.com>
Date: Sun, 2 Mar 2025 21:45:04 +0900
Subject: [PATCH 01/10] fix: parenthesize type in suggestion fixer if necessary

---
 .../src/rules/no-unnecessary-type-parameters.ts   | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/packages/eslint-plugin/src/rules/no-unnecessary-type-parameters.ts b/packages/eslint-plugin/src/rules/no-unnecessary-type-parameters.ts
index f66f75482289..2ad56ac1ca4e 100644
--- a/packages/eslint-plugin/src/rules/no-unnecessary-type-parameters.ts
+++ b/packages/eslint-plugin/src/rules/no-unnecessary-type-parameters.ts
@@ -108,7 +108,20 @@ export default createRule({
                 for (const reference of smTypeParameterVariable.references) {
                   if (reference.isTypeReference) {
                     const referenceNode = reference.identifier;
-                    yield fixer.replaceText(referenceNode, constraintText);
+                    const isUnionTypeArray =
+                      (constraint?.type === AST_NODE_TYPES.TSUnionType ||
+                        constraint?.type ===
+                          AST_NODE_TYPES.TSIntersectionType) &&
+                      referenceNode.parent.parent?.type ===
+                        AST_NODE_TYPES.TSArrayType;
+                    if (isUnionTypeArray) {
+                      yield fixer.replaceText(
+                        referenceNode,
+                        `(${constraintText})`,
+                      );
+                    } else {
+                      yield fixer.replaceText(referenceNode, constraintText);
+                    }
                   }
                 }
 

From e5cbe8e73c9428d693ab6d6efb33f9311bb2fd38 Mon Sep 17 00:00:00 2001
From: Hasegawa-Yukihiro <y.h.baskeeee@icloud.com>
Date: Sun, 2 Mar 2025 21:45:41 +0900
Subject: [PATCH 02/10] test: add test

---
 .../no-unnecessary-type-parameters.test.ts    | 46 +++++++++++++++++++
 1 file changed, 46 insertions(+)

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
index cba7279691f3..6cfce3cdd52b 100644
--- a/packages/eslint-plugin/tests/rules/no-unnecessary-type-parameters.test.ts
+++ b/packages/eslint-plugin/tests/rules/no-unnecessary-type-parameters.test.ts
@@ -1737,6 +1737,52 @@ class Joiner {
   join(els: 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>(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(',');
 }
       `,
             },

From cc8af2e763a46b323ec55009bd609af6628b5997 Mon Sep 17 00:00:00 2001
From: Hasegawa-Yukihiro <y.h.baskeeee@icloud.com>
Date: Sat, 8 Mar 2025 12:52:18 +0900
Subject: [PATCH 03/10] feat: support TSIndexedAccessType

---
 .../src/rules/no-unnecessary-type-parameters.ts   | 15 ++++++++-------
 1 file changed, 8 insertions(+), 7 deletions(-)

diff --git a/packages/eslint-plugin/src/rules/no-unnecessary-type-parameters.ts b/packages/eslint-plugin/src/rules/no-unnecessary-type-parameters.ts
index 2ad56ac1ca4e..9c05113e1621 100644
--- a/packages/eslint-plugin/src/rules/no-unnecessary-type-parameters.ts
+++ b/packages/eslint-plugin/src/rules/no-unnecessary-type-parameters.ts
@@ -108,13 +108,14 @@ export default createRule({
                 for (const reference of smTypeParameterVariable.references) {
                   if (reference.isTypeReference) {
                     const referenceNode = reference.identifier;
-                    const isUnionTypeArray =
-                      (constraint?.type === AST_NODE_TYPES.TSUnionType ||
-                        constraint?.type ===
-                          AST_NODE_TYPES.TSIntersectionType) &&
-                      referenceNode.parent.parent?.type ===
-                        AST_NODE_TYPES.TSArrayType;
-                    if (isUnionTypeArray) {
+                    const isCompositeType =
+                      constraint?.type === AST_NODE_TYPES.TSUnionType ||
+                      constraint?.type === AST_NODE_TYPES.TSIntersectionType;
+                    const hasMatchingAncestorType = [
+                      AST_NODE_TYPES.TSArrayType,
+                      AST_NODE_TYPES.TSIndexedAccessType,
+                    ].some(type => referenceNode.parent.parent?.type === type);
+                    if (isCompositeType && hasMatchingAncestorType) {
                       yield fixer.replaceText(
                         referenceNode,
                         `(${constraintText})`,

From 1a60c5beb4aaa59cf74005ace38953b5d10ff089 Mon Sep 17 00:00:00 2001
From: Hasegawa-Yukihiro <y.h.baskeeee@icloud.com>
Date: Sat, 8 Mar 2025 12:52:30 +0900
Subject: [PATCH 04/10] test: add test

---
 .../no-unnecessary-type-parameters.test.ts    | 23 +++++++++++++++++++
 1 file changed, 23 insertions(+)

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
index 6cfce3cdd52b..1322bc514d2d 100644
--- a/packages/eslint-plugin/tests/rules/no-unnecessary-type-parameters.test.ts
+++ b/packages/eslint-plugin/tests/rules/no-unnecessary-type-parameters.test.ts
@@ -1783,6 +1783,29 @@ function join<T extends string & number>(els: T[]) {
               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(',');
 }
       `,
             },

From 91dd50d9df0d4758deba94405cba78a6e2b4459e Mon Sep 17 00:00:00 2001
From: Hasegawa-Yukihiro <y.h.baskeeee@icloud.com>
Date: Mon, 10 Mar 2025 22:25:30 +0900
Subject: [PATCH 05/10] fix: add "!" type assertion

---
 .../eslint-plugin/src/rules/no-unnecessary-type-parameters.ts  | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/packages/eslint-plugin/src/rules/no-unnecessary-type-parameters.ts b/packages/eslint-plugin/src/rules/no-unnecessary-type-parameters.ts
index 9c05113e1621..581088dbbe1c 100644
--- a/packages/eslint-plugin/src/rules/no-unnecessary-type-parameters.ts
+++ b/packages/eslint-plugin/src/rules/no-unnecessary-type-parameters.ts
@@ -114,7 +114,8 @@ export default createRule({
                     const hasMatchingAncestorType = [
                       AST_NODE_TYPES.TSArrayType,
                       AST_NODE_TYPES.TSIndexedAccessType,
-                    ].some(type => referenceNode.parent.parent?.type === type);
+                      // eslint-disable-next-line @typescript-eslint/no-non-null-assertion
+                    ].some(type => referenceNode.parent.parent!.type === type);
                     if (isCompositeType && hasMatchingAncestorType) {
                       yield fixer.replaceText(
                         referenceNode,

From 4cb399a58302f85fd78a67688b17822e4640e219 Mon Sep 17 00:00:00 2001
From: Hasegawa-Yukihiro <y.h.baskeeee@icloud.com>
Date: Mon, 10 Mar 2025 23:08:13 +0900
Subject: [PATCH 06/10] test: add test

---
 .../no-unnecessary-type-parameters.test.ts    | 46 +++++++++++++++++++
 1 file changed, 46 insertions(+)

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
index 1322bc514d2d..aefaab0dcc14 100644
--- a/packages/eslint-plugin/tests/rules/no-unnecessary-type-parameters.test.ts
+++ b/packages/eslint-plugin/tests/rules/no-unnecessary-type-parameters.test.ts
@@ -1792,6 +1792,52 @@ function join(els: (string & number)[]) {
     },
     {
       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(',');
 }

From dd180515830952863dcc16aa360cd311522e138a Mon Sep 17 00:00:00 2001
From: Hasegawa-Yukihiro <y.h.baskeeee@icloud.com>
Date: Thu, 20 Mar 2025 14:28:49 +0900
Subject: [PATCH 07/10] fix: correct type precedence logic to consider broader
 context

---
 .../rules/no-unnecessary-type-parameters.ts   |  9 ++--
 .../no-unnecessary-type-parameters.test.ts    | 52 +++++++++++++++++++
 2 files changed, 58 insertions(+), 3 deletions(-)

diff --git a/packages/eslint-plugin/src/rules/no-unnecessary-type-parameters.ts b/packages/eslint-plugin/src/rules/no-unnecessary-type-parameters.ts
index 581088dbbe1c..257c541575b4 100644
--- a/packages/eslint-plugin/src/rules/no-unnecessary-type-parameters.ts
+++ b/packages/eslint-plugin/src/rules/no-unnecessary-type-parameters.ts
@@ -108,15 +108,18 @@ export default createRule({
                 for (const reference of smTypeParameterVariable.references) {
                   if (reference.isTypeReference) {
                     const referenceNode = reference.identifier;
-                    const isCompositeType =
+                    const isComplexType =
                       constraint?.type === AST_NODE_TYPES.TSUnionType ||
-                      constraint?.type === AST_NODE_TYPES.TSIntersectionType;
+                      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 (isCompositeType && hasMatchingAncestorType) {
+                    if (isComplexType && hasMatchingAncestorType) {
                       yield fixer.replaceText(
                         referenceNode,
                         `(${constraintText})`,
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
index aefaab0dcc14..9f8f05479c9c 100644
--- a/packages/eslint-plugin/tests/rules/no-unnecessary-type-parameters.test.ts
+++ b/packages/eslint-plugin/tests/rules/no-unnecessary-type-parameters.test.ts
@@ -1859,5 +1859,57 @@ function join(els: ({ hoge: string } | { hoge: number })['hoge'][]) {
         },
       ],
     },
+    {
+      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;
+      `,
+            },
+          ],
+        },
+      ],
+    },
   ],
 });

From 8505b5464a529d81238b64c8f5b1a8877b01bc91 Mon Sep 17 00:00:00 2001
From: Hasegawa-Yukihiro <y.h.baskeeee@icloud.com>
Date: Thu, 20 Mar 2025 16:32:38 +0900
Subject: [PATCH 08/10] fix: use getWrappingFixer

---
 .../rules/no-unnecessary-type-parameters.ts   | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/packages/eslint-plugin/src/rules/no-unnecessary-type-parameters.ts b/packages/eslint-plugin/src/rules/no-unnecessary-type-parameters.ts
index 257c541575b4..7296a0fa0a97 100644
--- 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';
@@ -120,10 +121,18 @@ export default createRule({
                       // eslint-disable-next-line @typescript-eslint/no-non-null-assertion
                     ].some(type => referenceNode.parent.parent!.type === type);
                     if (isComplexType && hasMatchingAncestorType) {
-                      yield fixer.replaceText(
-                        referenceNode,
-                        `(${constraintText})`,
-                      );
+                      const fixResult = getWrappingFixer({
+                        node: referenceNode,
+                        innerNode: constraint,
+                        sourceCode: context.sourceCode,
+                        wrap: constraintNode => constraintNode,
+                      })(fixer);
+                      if (fixResult == null) {
+                        return null;
+                      }
+                      if ('text' in fixResult) {
+                        yield fixResult;
+                      }
                     } else {
                       yield fixer.replaceText(referenceNode, constraintText);
                     }
@@ -140,7 +149,7 @@ export default createRule({
                 // We are assuming at this point that the reported type parameter
                 // is present in the inspected node's type parameters.
                 if (typeParamsNode.params.length === 1) {
-                  // Remove the whole <T> generic syntax if we're removing the only type parameter in the list.
+                  // Remove the whole <T> generic syntax if we're le <Ting the only type parameter in the list. we're removing the only type parameter in the list.
                   yield fixer.remove(typeParamsNode);
                 } else {
                   const index = typeParamsNode.params.indexOf(esTypeParameter);

From 80869243bbc6dfde53acb40f58d4001ff037245b Mon Sep 17 00:00:00 2001
From: Hasegawa-Yukihiro <y.h.baskeeee@icloud.com>
Date: Thu, 20 Mar 2025 16:34:36 +0900
Subject: [PATCH 09/10] chore: revert comment

---
 .../eslint-plugin/src/rules/no-unnecessary-type-parameters.ts   | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/packages/eslint-plugin/src/rules/no-unnecessary-type-parameters.ts b/packages/eslint-plugin/src/rules/no-unnecessary-type-parameters.ts
index 7296a0fa0a97..f02c92dfd006 100644
--- a/packages/eslint-plugin/src/rules/no-unnecessary-type-parameters.ts
+++ b/packages/eslint-plugin/src/rules/no-unnecessary-type-parameters.ts
@@ -149,7 +149,7 @@ export default createRule({
                 // We are assuming at this point that the reported type parameter
                 // is present in the inspected node's type parameters.
                 if (typeParamsNode.params.length === 1) {
-                  // Remove the whole <T> generic syntax if we're le <Ting the only type parameter in the list. we're removing the only type parameter in the list.
+                  // Remove the whole <T> generic syntax if we're removing the only type parameter in the list.
                   yield fixer.remove(typeParamsNode);
                 } else {
                   const index = typeParamsNode.params.indexOf(esTypeParameter);

From 66a498861630f4bfffcef8d9986f75154b83ac37 Mon Sep 17 00:00:00 2001
From: Kirk Waiblinger <53019676+kirkwaiblinger@users.noreply.github.com>
Date: Mon, 28 Apr 2025 14:18:23 -0600
Subject: [PATCH 10/10] address cov

---
 .../src/rules/no-unnecessary-type-parameters.ts            | 7 +------
 packages/eslint-plugin/src/util/getWrappingFixer.ts        | 2 +-
 2 files changed, 2 insertions(+), 7 deletions(-)

diff --git a/packages/eslint-plugin/src/rules/no-unnecessary-type-parameters.ts b/packages/eslint-plugin/src/rules/no-unnecessary-type-parameters.ts
index f02c92dfd006..3a486639dff7 100644
--- a/packages/eslint-plugin/src/rules/no-unnecessary-type-parameters.ts
+++ b/packages/eslint-plugin/src/rules/no-unnecessary-type-parameters.ts
@@ -127,12 +127,7 @@ export default createRule({
                         sourceCode: context.sourceCode,
                         wrap: constraintNode => constraintNode,
                       })(fixer);
-                      if (fixResult == null) {
-                        return null;
-                      }
-                      if ('text' in fixResult) {
-                        yield fixResult;
-                      }
+                      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
index 26afcaa6405b..2ad20e1d4893 100644
--- 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];
 

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