Don't widen when type parameter occurs at top level in type predicate

src/compiler/checker.ts

@@ -23382,6 +23382,12 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
             type.flags & TypeFlags.Conditional && (getTrueTypeFromConditionalType(type as ConditionalType) === typeParameter || getFalseTypeFromConditionalType(type as ConditionalType) === typeParameter));
     }
 
+    function isTypeParameterAtTopLevelInReturnType(signature: Signature, typeParameter: TypeParameter) {
+        const typePredicate = getTypePredicateOfSignature(signature);
+        return typePredicate ? !!typePredicate.type && isTypeParameterAtTopLevel(typePredicate.type, typeParameter) :
+            isTypeParameterAtTopLevel(getReturnTypeOfSignature(signature), typeParameter);
+    }
+
     /** Create an object with properties named in the string literal type. Every property has type `any` */
     function createEmptyObjectTypeFromStringLiteral(type: Type) {
         const members = createSymbolTable();
@@ -24458,7 +24464,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
         // the type parameter was fixed during inference or does not occur at top-level in the return type.
         const primitiveConstraint = hasPrimitiveConstraint(inference.typeParameter) || isConstTypeVariable(inference.typeParameter);
         const widenLiteralTypes = !primitiveConstraint && inference.topLevel &&
-            (inference.isFixed || !isTypeParameterAtTopLevel(getReturnTypeOfSignature(signature), inference.typeParameter));
+            (inference.isFixed || !isTypeParameterAtTopLevelInReturnType(signature, inference.typeParameter));
         const baseCandidates = primitiveConstraint ? sameMap(candidates, getRegularTypeOfLiteralType) :
             widenLiteralTypes ? sameMap(candidates, getWidenedLiteralType) :
             candidates;

tests/baselines/reference/typePredicateTopLevelTypeParameter.js

@@ -0,0 +1,30 @@
+//// [typePredicateTopLevelTypeParameter.ts]
+// Repro from #51980
+
+function getPermissions(user: string) {
+    if (user === 'Jack') return 'admin';
+    return undefined;
+}
+
+const admins = ['Mike', 'Joe'].map(e => getPermissions(e));
+
+function isDefined<T>(a: T | undefined): a is T {
+    return a !== undefined;
+}
+
+const foundAdmins = admins.filter(isDefined);  // "admin"[]
+
+
+//// [typePredicateTopLevelTypeParameter.js]
+"use strict";
+// Repro from #51980
+function getPermissions(user) {
+    if (user === 'Jack')
+        return 'admin';
+    return undefined;
+}
+var admins = ['Mike', 'Joe'].map(function (e) { return getPermissions(e); });
+function isDefined(a) {
+    return a !== undefined;
+}
+var foundAdmins = admins.filter(isDefined); // "admin"[]

tests/baselines/reference/typePredicateTopLevelTypeParameter.symbols

@@ -0,0 +1,42 @@
+=== tests/cases/compiler/typePredicateTopLevelTypeParameter.ts ===
+// Repro from #51980
+
+function getPermissions(user: string) {
+>getPermissions : Symbol(getPermissions, Decl(typePredicateTopLevelTypeParameter.ts, 0, 0))
+>user : Symbol(user, Decl(typePredicateTopLevelTypeParameter.ts, 2, 24))
+
+    if (user === 'Jack') return 'admin';
+>user : Symbol(user, Decl(typePredicateTopLevelTypeParameter.ts, 2, 24))
+
+    return undefined;
+>undefined : Symbol(undefined)
+}
+
+const admins = ['Mike', 'Joe'].map(e => getPermissions(e));
+>admins : Symbol(admins, Decl(typePredicateTopLevelTypeParameter.ts, 7, 5))
+>['Mike', 'Joe'].map : Symbol(Array.map, Decl(lib.es5.d.ts, --, --))
+>map : Symbol(Array.map, Decl(lib.es5.d.ts, --, --))
+>e : Symbol(e, Decl(typePredicateTopLevelTypeParameter.ts, 7, 35))
+>getPermissions : Symbol(getPermissions, Decl(typePredicateTopLevelTypeParameter.ts, 0, 0))
+>e : Symbol(e, Decl(typePredicateTopLevelTypeParameter.ts, 7, 35))
+
+function isDefined<T>(a: T | undefined): a is T {
+>isDefined : Symbol(isDefined, Decl(typePredicateTopLevelTypeParameter.ts, 7, 59))
+>T : Symbol(T, Decl(typePredicateTopLevelTypeParameter.ts, 9, 19))
+>a : Symbol(a, Decl(typePredicateTopLevelTypeParameter.ts, 9, 22))
+>T : Symbol(T, Decl(typePredicateTopLevelTypeParameter.ts, 9, 19))
+>a : Symbol(a, Decl(typePredicateTopLevelTypeParameter.ts, 9, 22))
+>T : Symbol(T, Decl(typePredicateTopLevelTypeParameter.ts, 9, 19))
+
+    return a !== undefined;
+>a : Symbol(a, Decl(typePredicateTopLevelTypeParameter.ts, 9, 22))
+>undefined : Symbol(undefined)
+}
+
+const foundAdmins = admins.filter(isDefined);  // "admin"[]
+>foundAdmins : Symbol(foundAdmins, Decl(typePredicateTopLevelTypeParameter.ts, 13, 5))
+>admins.filter : Symbol(Array.filter, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>admins : Symbol(admins, Decl(typePredicateTopLevelTypeParameter.ts, 7, 5))
+>filter : Symbol(Array.filter, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>isDefined : Symbol(isDefined, Decl(typePredicateTopLevelTypeParameter.ts, 7, 59))
+

tests/baselines/reference/typePredicateTopLevelTypeParameter.types

@@ -0,0 +1,49 @@
+=== tests/cases/compiler/typePredicateTopLevelTypeParameter.ts ===
+// Repro from #51980
+
+function getPermissions(user: string) {
+>getPermissions : (user: string) => "admin" | undefined
+>user : string
+
+    if (user === 'Jack') return 'admin';
+>user === 'Jack' : boolean
+>user : string
+>'Jack' : "Jack"
+>'admin' : "admin"
+
+    return undefined;
+>undefined : undefined
+}
+
+const admins = ['Mike', 'Joe'].map(e => getPermissions(e));
+>admins : ("admin" | undefined)[]
+>['Mike', 'Joe'].map(e => getPermissions(e)) : ("admin" | undefined)[]
+>['Mike', 'Joe'].map : <U>(callbackfn: (value: string, index: number, array: string[]) => U, thisArg?: any) => U[]
+>['Mike', 'Joe'] : string[]
+>'Mike' : "Mike"
+>'Joe' : "Joe"
+>map : <U>(callbackfn: (value: string, index: number, array: string[]) => U, thisArg?: any) => U[]
+>e => getPermissions(e) : (e: string) => "admin" | undefined
+>e : string
+>getPermissions(e) : "admin" | undefined
+>getPermissions : (user: string) => "admin" | undefined
+>e : string
+
+function isDefined<T>(a: T | undefined): a is T {
+>isDefined : <T>(a: T | undefined) => a is T
+>a : T | undefined
+
+    return a !== undefined;
+>a !== undefined : boolean
+>a : T | undefined
+>undefined : undefined
+}
+
+const foundAdmins = admins.filter(isDefined);  // "admin"[]
+>foundAdmins : "admin"[]
+>admins.filter(isDefined) : "admin"[]
+>admins.filter : { <S extends "admin" | undefined>(predicate: (value: "admin" | undefined, index: number, array: ("admin" | undefined)[]) => value is S, thisArg?: any): S[]; (predicate: (value: "admin" | undefined, index: number, array: ("admin" | undefined)[]) => unknown, thisArg?: any): ("admin" | undefined)[]; }
+>admins : ("admin" | undefined)[]
+>filter : { <S extends "admin" | undefined>(predicate: (value: "admin" | undefined, index: number, array: ("admin" | undefined)[]) => value is S, thisArg?: any): S[]; (predicate: (value: "admin" | undefined, index: number, array: ("admin" | undefined)[]) => unknown, thisArg?: any): ("admin" | undefined)[]; }
+>isDefined : <T>(a: T | undefined) => a is T
+

tests/cases/compiler/typePredicateTopLevelTypeParameter.ts

@@ -0,0 +1,16 @@
+// @strict: true
+
+// Repro from #51980
+
+function getPermissions(user: string) {
+    if (user === 'Jack') return 'admin';
+    return undefined;
+}
+
+const admins = ['Mike', 'Joe'].map(e => getPermissions(e));
+
+function isDefined<T>(a: T | undefined): a is T {
+    return a !== undefined;
+}
+
+const foundAdmins = admins.filter(isDefined);  // "admin"[]