From 4289adb5a0ea8d3500680f382174e4efbe599e82 Mon Sep 17 00:00:00 2001
From: Josh Goldberg <git@joshuakgoldberg.com>
Date: Tue, 6 Aug 2024 15:49:59 -0400
Subject: [PATCH 1/3] fix(eslint-plugin): [no-unnecessary-type-parameters]
 check mapped alias type arguments

---
 .../rules/no-unnecessary-type-parameters.ts    |  1 +
 .../no-unnecessary-type-parameters.test.ts     | 18 ++++++++++++++++++
 2 files changed, 19 insertions(+)

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 f191efa2f4e7..77cb6f931962 100644
--- a/packages/eslint-plugin/src/rules/no-unnecessary-type-parameters.ts
+++ b/packages/eslint-plugin/src/rules/no-unnecessary-type-parameters.ts
@@ -265,6 +265,7 @@ function collectTypeParameterUsageCounts(
     else if (tsutils.isConditionalType(type)) {
       visitType(type.checkType, assumeMultipleUses);
       visitType(type.extendsType, assumeMultipleUses);
+      type.aliasTypeArguments?.forEach(typeArgument => visitType(typeArgument, false));
     }
 
     // Catch-all: inferred object types like `{ K: V }`.
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 e5a7558e2f49..64bbece72f2f 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
@@ -342,6 +342,24 @@ ruleTester.run('no-unnecessary-type-parameters', rule, {
         (token): token is Exclude<TSESTree.Token, Conditions & ExtractedToken> =>
           tokenType in conditions;
     `,
+    `
+      type Foo<T, S> = S extends 'somebody'
+        ? T extends 'once'
+          ? 'told'
+          : 'me'
+        : never;
+
+      declare function foo<T>(data: T): <S>(other: S) => Foo<T, S>;
+    `,
+    `
+      type Foo<T, S> = S extends 'somebody'
+        ? T extends 'once'
+          ? 'told'
+          : 'me'
+        : never;
+
+      declare function foo<T>(data: T): <S>(other: S) => Foo<S, T>;
+    `,
     `
       declare function mapObj<K extends string, V>(
         obj: { [key in K]?: V },

From 469e9337e519c80916a472108c2526734d86227f Mon Sep 17 00:00:00 2001
From: Josh Goldberg <git@joshuakgoldberg.com>
Date: Mon, 12 Aug 2024 15:32:46 -0400
Subject: [PATCH 2/3] yarn format --write

---
 .../eslint-plugin/src/rules/no-unnecessary-type-parameters.ts | 4 +++-
 1 file changed, 3 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 0ce98d4395bd..ebee4ef0015e 100644
--- a/packages/eslint-plugin/src/rules/no-unnecessary-type-parameters.ts
+++ b/packages/eslint-plugin/src/rules/no-unnecessary-type-parameters.ts
@@ -288,7 +288,9 @@ function collectTypeParameterUsageCounts(
     else if (tsutils.isConditionalType(type)) {
       visitType(type.checkType, assumeMultipleUses);
       visitType(type.extendsType, assumeMultipleUses);
-      type.aliasTypeArguments?.forEach(typeArgument => visitType(typeArgument, false));
+      type.aliasTypeArguments?.forEach(typeArgument =>
+        visitType(typeArgument, false),
+      );
     }
 
     // Catch-all: inferred object types like `{ K: V }`.

From cc6563cf418a1c51b69db14664e873eed666a61b Mon Sep 17 00:00:00 2001
From: Dan Vanderkam <danvdk@gmail.com>
Date: Mon, 19 Aug 2024 12:29:57 -0400
Subject: [PATCH 3/3] Handle type aliases higher up in #9741 (#323)

handle type aliases higher up
---
 .../rules/no-unnecessary-type-parameters.ts   | 19 +++++++------------
 1 file changed, 7 insertions(+), 12 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 ebee4ef0015e..6b74879b0dfe 100644
--- a/packages/eslint-plugin/src/rules/no-unnecessary-type-parameters.ts
+++ b/packages/eslint-plugin/src/rules/no-unnecessary-type-parameters.ts
@@ -258,6 +258,13 @@ function collectTypeParameterUsageCounts(
       }
     }
 
+    // Catch-all: generic type references like `Exclude<T, null>`
+    else if (type.aliasTypeArguments) {
+      // We don't descend into the definition of the type alias, so we don't
+      // know whether it's used multiple times. It's safest to assume it is.
+      visitTypesList(type.aliasTypeArguments, true);
+    }
+
     // Intersections and unions like `0 | 1`
     else if (tsutils.isUnionOrIntersectionType(type)) {
       visitTypesList(type.types, assumeMultipleUses);
@@ -288,9 +295,6 @@ function collectTypeParameterUsageCounts(
     else if (tsutils.isConditionalType(type)) {
       visitType(type.checkType, assumeMultipleUses);
       visitType(type.extendsType, assumeMultipleUses);
-      type.aliasTypeArguments?.forEach(typeArgument =>
-        visitType(typeArgument, false),
-      );
     }
 
     // Catch-all: inferred object types like `{ K: V }`.
@@ -310,10 +314,6 @@ function collectTypeParameterUsageCounts(
         }
       }
 
-      for (const typeArgument of type.aliasTypeArguments ?? []) {
-        visitType(typeArgument, true);
-      }
-
       visitType(type.getNumberIndexType(), true);
       visitType(type.getStringIndexType(), true);
 
@@ -332,11 +332,6 @@ function collectTypeParameterUsageCounts(
     else if (isOperatorType(type)) {
       visitType(type.type, assumeMultipleUses);
     }
-
-    // Catch-all: generic type references like `Exclude<T, null>`
-    else if (type.aliasTypeArguments) {
-      visitTypesList(type.aliasTypeArguments, true);
-    }
   }
 
   function incrementIdentifierCount(

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