Favor asserted type in type predicate narrowing (#50044)

* Favor asserted type in type predicate narrowing

* Accept new baselines

Author: ahejlsberg

src/compiler/checker.ts

@@ -25428,8 +25428,12 @@ namespace ts {
                     const discriminant = keyPropertyName && getTypeOfPropertyOfType(c, keyPropertyName);
                     const matching = discriminant && getConstituentTypeForKeyType(type as UnionType, discriminant);
                     // For each constituent t in the current type, if t and and c are directly related, pick the most
-                    // specific of the two.
-                    const directlyRelated = mapType(matching || type, t => isRelated(t, c) ? t : isRelated(c, t) ? c : neverType);
+                    // specific of the two. When t and c are related in both directions, we prefer c for type predicates
+                    // because that is the asserted type, but t for `instanceof` because generics aren't reflected in
+                    // prototype object types.
+                    const directlyRelated = mapType(matching || type, checkDerived ?
+                        t => isTypeDerivedFrom(t, c) ? t : isTypeDerivedFrom(c, t) ? c : neverType :
+                        t => isTypeSubtypeOf(c, t) ? c : isTypeSubtypeOf(t, c) ? t : neverType);
                     // If no constituents are directly related, create intersections for any generic constituents that
                     // are related by constraint.
                     return directlyRelated.flags & TypeFlags.Never ?

tests/baselines/reference/controlFlowBinaryOrExpression.symbols

@@ -64,19 +64,19 @@ if (isNodeList(sourceObj)) {
 >sourceObj : Symbol(sourceObj, Decl(controlFlowBinaryOrExpression.ts, 23, 3))
 
     sourceObj.length;
->sourceObj.length : Symbol(length, Decl(controlFlowBinaryOrExpression.ts, 10, 27), Decl(controlFlowBinaryOrExpression.ts, 14, 33))
+>sourceObj.length : Symbol(NodeList.length, Decl(controlFlowBinaryOrExpression.ts, 10, 27))
 >sourceObj : Symbol(sourceObj, Decl(controlFlowBinaryOrExpression.ts, 23, 3))
->length : Symbol(length, Decl(controlFlowBinaryOrExpression.ts, 10, 27), Decl(controlFlowBinaryOrExpression.ts, 14, 33))
+>length : Symbol(NodeList.length, Decl(controlFlowBinaryOrExpression.ts, 10, 27))
 }
 
 if (isHTMLCollection(sourceObj)) {
 >isHTMLCollection : Symbol(isHTMLCollection, Decl(controlFlowBinaryOrExpression.ts, 18, 67))
 >sourceObj : Symbol(sourceObj, Decl(controlFlowBinaryOrExpression.ts, 23, 3))
 
     sourceObj.length;
->sourceObj.length : Symbol(length, Decl(controlFlowBinaryOrExpression.ts, 10, 27), Decl(controlFlowBinaryOrExpression.ts, 14, 33))
+>sourceObj.length : Symbol(HTMLCollection.length, Decl(controlFlowBinaryOrExpression.ts, 14, 33))
 >sourceObj : Symbol(sourceObj, Decl(controlFlowBinaryOrExpression.ts, 23, 3))
->length : Symbol(length, Decl(controlFlowBinaryOrExpression.ts, 10, 27), Decl(controlFlowBinaryOrExpression.ts, 14, 33))
+>length : Symbol(HTMLCollection.length, Decl(controlFlowBinaryOrExpression.ts, 14, 33))
 }
 
 if (isNodeList(sourceObj) || isHTMLCollection(sourceObj)) {

tests/baselines/reference/controlFlowBinaryOrExpression.types

@@ -69,26 +69,26 @@ if (isNodeList(sourceObj)) {
 
     sourceObj.length;
 >sourceObj.length : number
->sourceObj : NodeList | HTMLCollection
+>sourceObj : NodeList
 >length : number
 }
 
 if (isHTMLCollection(sourceObj)) {
 >isHTMLCollection(sourceObj) : boolean
 >isHTMLCollection : (sourceObj: any) => sourceObj is HTMLCollection
->sourceObj : EventTargetLike
+>sourceObj : NodeList | { a: string; }
 
     sourceObj.length;
 >sourceObj.length : number
->sourceObj : NodeList | HTMLCollection
+>sourceObj : HTMLCollection
 >length : number
 }
 
 if (isNodeList(sourceObj) || isHTMLCollection(sourceObj)) {
 >isNodeList(sourceObj) || isHTMLCollection(sourceObj) : boolean
 >isNodeList(sourceObj) : boolean
 >isNodeList : (sourceObj: any) => sourceObj is NodeList
->sourceObj : EventTargetLike
+>sourceObj : HTMLCollection | { a: string; }
 >isHTMLCollection(sourceObj) : boolean
 >isHTMLCollection : (sourceObj: any) => sourceObj is HTMLCollection
 >sourceObj : { a: string; }

tests/baselines/reference/instanceofWithStructurallyIdenticalTypes.symbols

@@ -95,9 +95,9 @@ function foo2(x: C1 | C2 | C3): string {
 >x : Symbol(x, Decl(instanceofWithStructurallyIdenticalTypes.ts, 23, 14))
 
         return x.item;
->x.item : Symbol(item, Decl(instanceofWithStructurallyIdenticalTypes.ts, 2, 10), Decl(instanceofWithStructurallyIdenticalTypes.ts, 4, 10))
+>x.item : Symbol(C1.item, Decl(instanceofWithStructurallyIdenticalTypes.ts, 2, 10))
 >x : Symbol(x, Decl(instanceofWithStructurallyIdenticalTypes.ts, 23, 14))
->item : Symbol(item, Decl(instanceofWithStructurallyIdenticalTypes.ts, 2, 10), Decl(instanceofWithStructurallyIdenticalTypes.ts, 4, 10))
+>item : Symbol(C1.item, Decl(instanceofWithStructurallyIdenticalTypes.ts, 2, 10))
     }
     else if (isC2(x)) {
 >isC2 : Symbol(isC2, Decl(instanceofWithStructurallyIdenticalTypes.ts, 19, 66))

tests/baselines/reference/instanceofWithStructurallyIdenticalTypes.types

@@ -85,7 +85,7 @@ function foo2(x: C1 | C2 | C3): string {
 
         return x.item;
 >x.item : string
->x : C1 | C3
+>x : C1
 >item : string
     }
     else if (isC2(x)) {

tests/baselines/reference/typePredicateStructuralMatch.symbols

@@ -51,9 +51,9 @@ function getResults1(value: Results | { data: Results }): Results {
     return isResponseInData(value) ? value.data : value;
 >isResponseInData : Symbol(isResponseInData, Decl(typePredicateStructuralMatch.ts, 9, 24))
 >value : Symbol(value, Decl(typePredicateStructuralMatch.ts, 15, 21))
->value.data : Symbol(data, Decl(typePredicateStructuralMatch.ts, 15, 39))
+>value.data : Symbol(data, Decl(typePredicateStructuralMatch.ts, 11, 63))
 >value : Symbol(value, Decl(typePredicateStructuralMatch.ts, 15, 21))
->data : Symbol(data, Decl(typePredicateStructuralMatch.ts, 15, 39))
+>data : Symbol(data, Decl(typePredicateStructuralMatch.ts, 11, 63))
 >value : Symbol(value, Decl(typePredicateStructuralMatch.ts, 15, 21))
 }
 

tests/baselines/reference/unionWithIndexSignature.types

@@ -55,7 +55,7 @@ export function flatten<T extends number|TypedArray>(arr: T) {
 
       arr[1];
 >arr[1] : number
->arr : TypedArray
+>arr : Int32Array | Uint8Array
 >1 : 1
   }
 }