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 02b51d184533..3a36457a334b 100644 --- a/packages/eslint-plugin/src/rules/no-unnecessary-type-parameters.ts +++ b/packages/eslint-plugin/src/rules/no-unnecessary-type-parameters.ts @@ -303,7 +303,7 @@ function collectTypeParameterUsageCounts( if (properties.length === 0) { // TS treats mapped types like `{[k in "a"]: T}` like `{a: T}`. // They have properties, so we need to avoid double-counting. - visitType(type.templateType, false); + visitType(type.templateType ?? type.constraintType, false); } } 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 1ad76a9210d7..0fd90f14658f 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 @@ -348,6 +348,33 @@ ruleTester.run('no-unnecessary-type-parameters', rule, { fn: (key: K, val: V) => number, ): number[]; `, + ` + declare function mappedReturnType( + x: T, + ): { [K in T]: Capitalize }; + + function inferredMappedReturnType(x: T) { + return mappedReturnType(x); + } + `, + ` + declare function mappedReturnType( + x: T, + ): { [K in T]: Capitalize }; + + function inferredMappedReturnType(x: T) { + return () => mappedReturnType(x); + } + `, + ` + declare function mappedReturnType( + x: T, + ): { [K in T]: Capitalize }; + + function inferredMappedReturnType(x: T) { + return [{ value: () => mappedReturnType(x) }]; + } + `, ], invalid: [ pFad - Phonifier reborn

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