fix(eslint-plugin): [no-unnecessary-type-parameters] check mapped alias type arguments (typescript-eslint#9741)

JoshuaKGoldberg · danvk · web-flow · commit 94f7c99550aa · 2024-08-20T02:21:23.000-04:00
* fix(eslint-plugin): [no-unnecessary-type-parameters] check mapped alias type arguments * yarn format --write * Handle type aliases higher up in typescript-eslint#9741 (typescript-eslint#323) handle type aliases higher up --------- Co-authored-by: Dan Vanderkam <danvdk@gmail.com>
diff --git 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);
@@ -307,10 +314,6 @@ function collectTypeParameterUsageCounts(
         }
       }
 
-      for (const typeArgument of type.aliasTypeArguments ?? []) {
-        visitType(typeArgument, true);
-      }
-
       visitType(type.getNumberIndexType(), true);
       visitType(type.getStringIndexType(), true);
 
@@ -329,11 +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(
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
@@ -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 },

Original file line number	Diff line number	Diff line change
`@@ -258,6 +258,13 @@ function collectTypeParameterUsageCounts(`
`258`	`258`	`}`
`259`	`259`	`}`
`260`	`260`
	`261`	+ // Catch-all: generic type references like `Exclude<T, null>`
	`262`	`+ else if (type.aliasTypeArguments) {`
	`263`	`+ // We don't descend into the definition of the type alias, so we don't`
	`264`	`+ // know whether it's used multiple times. It's safest to assume it is.`
	`265`	`+ visitTypesList(type.aliasTypeArguments, true);`
	`266`	`+ }`
	`267`	`+`
`261`	`268`	// Intersections and unions like `0 \| 1`
`262`	`269`	`else if (tsutils.isUnionOrIntersectionType(type)) {`
`263`	`270`	`visitTypesList(type.types, assumeMultipleUses);`
`@@ -307,10 +314,6 @@ function collectTypeParameterUsageCounts(`
`307`	`314`	`}`
`308`	`315`	`}`
`309`	`316`
`310`		`- for (const typeArgument of type.aliasTypeArguments ?? []) {`
`311`		`- visitType(typeArgument, true);`
`312`		`- }`
`313`		`-`
`314`	`317`	`visitType(type.getNumberIndexType(), true);`
`315`	`318`	`visitType(type.getStringIndexType(), true);`
`316`	`319`
`@@ -329,11 +332,6 @@ function collectTypeParameterUsageCounts(`
`329`	`332`	`else if (isOperatorType(type)) {`
`330`	`333`	`visitType(type.type, assumeMultipleUses);`
`331`	`334`	`}`
`332`		`-`
`333`		- // Catch-all: generic type references like `Exclude<T, null>`
`334`		`- else if (type.aliasTypeArguments) {`
`335`		`- visitTypesList(type.aliasTypeArguments, true);`
`336`		`- }`
`337`	`335`	`}`
`338`	`336`
`339`	`337`	`function incrementIdentifierCount(`