Fix check in getDiscriminantPropertyAccess (#55464)

src/compiler/checker.ts

@@ -27460,7 +27460,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
         }
 
         function getDiscriminantPropertyAccess(expr: Expression, computedType: Type) {
-            const type = declaredType.flags & TypeFlags.Union ? declaredType : computedType;
+            const type = !(computedType.flags & TypeFlags.Union) && declaredType.flags & TypeFlags.Union ? declaredType : computedType;
             if (type.flags & TypeFlags.Union) {
                 const access = getCandidateDiscriminantPropertyAccess(expr);
                 if (access) {

tests/baselines/reference/narrowingUnionToUnion.js

@@ -212,6 +212,20 @@ function f1x(obj: (string | number)[] | null) {
     assertRelationIsNullOrStringArray(obj);
     obj;  // string[] | null
 }
+
+// Repro from #55425
+
+type MyDiscriminatedUnion = { type: 'A', aProp: number } | { type: 'B', bProp: string };
+
+declare function isMyDiscriminatedUnion(item: unknown): item is MyDiscriminatedUnion;
+
+declare const working: unknown;
+declare const broken: Record<string, any> | undefined;
+declare const workingAgain: Record<string, any> | undefined | unknown;
+
+isMyDiscriminatedUnion(working) && working.type === 'A' && working.aProp;
+isMyDiscriminatedUnion(broken) && broken.type === 'A' && broken.aProp;
+isMyDiscriminatedUnion(workingAgain) && workingAgain.type === 'A' && workingAgain.aProp;
 
 
 //// [narrowingUnionToUnion.js]
@@ -370,6 +384,9 @@ function f1x(obj) {
     assertRelationIsNullOrStringArray(obj);
     obj; // string[] | null
 }
+isMyDiscriminatedUnion(working) && working.type === 'A' && working.aProp;
+isMyDiscriminatedUnion(broken) && broken.type === 'A' && broken.aProp;
+isMyDiscriminatedUnion(workingAgain) && workingAgain.type === 'A' && workingAgain.aProp;
 
 
 //// [narrowingUnionToUnion.d.ts]
@@ -423,3 +440,14 @@ declare function check2(x: unknown): x is ("hello" | 0);
 declare function test3(x: unknown): void;
 declare function assertRelationIsNullOrStringArray(v: (string | number)[] | null): asserts v is string[] | null;
 declare function f1x(obj: (string | number)[] | null): void;
+type MyDiscriminatedUnion = {
+    type: 'A';
+    aProp: number;
+} | {
+    type: 'B';
+    bProp: string;
+};
+declare function isMyDiscriminatedUnion(item: unknown): item is MyDiscriminatedUnion;
+declare const working: unknown;
+declare const broken: Record<string, any> | undefined;
+declare const workingAgain: Record<string, any> | undefined | unknown;

tests/baselines/reference/narrowingUnionToUnion.symbols

@@ -482,3 +482,59 @@ function f1x(obj: (string | number)[] | null) {
 >obj : Symbol(obj, Decl(narrowingUnionToUnion.ts, 207, 13))
 }
 
+// Repro from #55425
+
+type MyDiscriminatedUnion = { type: 'A', aProp: number } | { type: 'B', bProp: string };
+>MyDiscriminatedUnion : Symbol(MyDiscriminatedUnion, Decl(narrowingUnionToUnion.ts, 210, 1))
+>type : Symbol(type, Decl(narrowingUnionToUnion.ts, 214, 29))
+>aProp : Symbol(aProp, Decl(narrowingUnionToUnion.ts, 214, 40))
+>type : Symbol(type, Decl(narrowingUnionToUnion.ts, 214, 60))
+>bProp : Symbol(bProp, Decl(narrowingUnionToUnion.ts, 214, 71))
+
+declare function isMyDiscriminatedUnion(item: unknown): item is MyDiscriminatedUnion;
+>isMyDiscriminatedUnion : Symbol(isMyDiscriminatedUnion, Decl(narrowingUnionToUnion.ts, 214, 88))
+>item : Symbol(item, Decl(narrowingUnionToUnion.ts, 216, 40))
+>item : Symbol(item, Decl(narrowingUnionToUnion.ts, 216, 40))
+>MyDiscriminatedUnion : Symbol(MyDiscriminatedUnion, Decl(narrowingUnionToUnion.ts, 210, 1))
+
+declare const working: unknown;
+>working : Symbol(working, Decl(narrowingUnionToUnion.ts, 218, 13))
+
+declare const broken: Record<string, any> | undefined;
+>broken : Symbol(broken, Decl(narrowingUnionToUnion.ts, 219, 13))
+>Record : Symbol(Record, Decl(lib.es5.d.ts, --, --))
+
+declare const workingAgain: Record<string, any> | undefined | unknown;
+>workingAgain : Symbol(workingAgain, Decl(narrowingUnionToUnion.ts, 220, 13))
+>Record : Symbol(Record, Decl(lib.es5.d.ts, --, --))
+
+isMyDiscriminatedUnion(working) && working.type === 'A' && working.aProp;
+>isMyDiscriminatedUnion : Symbol(isMyDiscriminatedUnion, Decl(narrowingUnionToUnion.ts, 214, 88))
+>working : Symbol(working, Decl(narrowingUnionToUnion.ts, 218, 13))
+>working.type : Symbol(type, Decl(narrowingUnionToUnion.ts, 214, 29), Decl(narrowingUnionToUnion.ts, 214, 60))
+>working : Symbol(working, Decl(narrowingUnionToUnion.ts, 218, 13))
+>type : Symbol(type, Decl(narrowingUnionToUnion.ts, 214, 29), Decl(narrowingUnionToUnion.ts, 214, 60))
+>working.aProp : Symbol(aProp, Decl(narrowingUnionToUnion.ts, 214, 40))
+>working : Symbol(working, Decl(narrowingUnionToUnion.ts, 218, 13))
+>aProp : Symbol(aProp, Decl(narrowingUnionToUnion.ts, 214, 40))
+
+isMyDiscriminatedUnion(broken) && broken.type === 'A' && broken.aProp;
+>isMyDiscriminatedUnion : Symbol(isMyDiscriminatedUnion, Decl(narrowingUnionToUnion.ts, 214, 88))
+>broken : Symbol(broken, Decl(narrowingUnionToUnion.ts, 219, 13))
+>broken.type : Symbol(type, Decl(narrowingUnionToUnion.ts, 214, 29), Decl(narrowingUnionToUnion.ts, 214, 60))
+>broken : Symbol(broken, Decl(narrowingUnionToUnion.ts, 219, 13))
+>type : Symbol(type, Decl(narrowingUnionToUnion.ts, 214, 29), Decl(narrowingUnionToUnion.ts, 214, 60))
+>broken.aProp : Symbol(aProp, Decl(narrowingUnionToUnion.ts, 214, 40))
+>broken : Symbol(broken, Decl(narrowingUnionToUnion.ts, 219, 13))
+>aProp : Symbol(aProp, Decl(narrowingUnionToUnion.ts, 214, 40))
+
+isMyDiscriminatedUnion(workingAgain) && workingAgain.type === 'A' && workingAgain.aProp;
+>isMyDiscriminatedUnion : Symbol(isMyDiscriminatedUnion, Decl(narrowingUnionToUnion.ts, 214, 88))
+>workingAgain : Symbol(workingAgain, Decl(narrowingUnionToUnion.ts, 220, 13))
+>workingAgain.type : Symbol(type, Decl(narrowingUnionToUnion.ts, 214, 29), Decl(narrowingUnionToUnion.ts, 214, 60))
+>workingAgain : Symbol(workingAgain, Decl(narrowingUnionToUnion.ts, 220, 13))
+>type : Symbol(type, Decl(narrowingUnionToUnion.ts, 214, 29), Decl(narrowingUnionToUnion.ts, 214, 60))
+>workingAgain.aProp : Symbol(aProp, Decl(narrowingUnionToUnion.ts, 214, 40))
+>workingAgain : Symbol(workingAgain, Decl(narrowingUnionToUnion.ts, 220, 13))
+>aProp : Symbol(aProp, Decl(narrowingUnionToUnion.ts, 214, 40))
+

tests/baselines/reference/narrowingUnionToUnion.types

@@ -507,3 +507,70 @@ function f1x(obj: (string | number)[] | null) {
 >obj : string[] | null
 }
 
+// Repro from #55425
+
+type MyDiscriminatedUnion = { type: 'A', aProp: number } | { type: 'B', bProp: string };
+>MyDiscriminatedUnion : { type: 'A'; aProp: number; } | { type: 'B'; bProp: string; }
+>type : "A"
+>aProp : number
+>type : "B"
+>bProp : string
+
+declare function isMyDiscriminatedUnion(item: unknown): item is MyDiscriminatedUnion;
+>isMyDiscriminatedUnion : (item: unknown) => item is MyDiscriminatedUnion
+>item : unknown
+
+declare const working: unknown;
+>working : unknown
+
+declare const broken: Record<string, any> | undefined;
+>broken : Record<string, any> | undefined
+
+declare const workingAgain: Record<string, any> | undefined | unknown;
+>workingAgain : unknown
+
+isMyDiscriminatedUnion(working) && working.type === 'A' && working.aProp;
+>isMyDiscriminatedUnion(working) && working.type === 'A' && working.aProp : number | false
+>isMyDiscriminatedUnion(working) && working.type === 'A' : boolean
+>isMyDiscriminatedUnion(working) : boolean
+>isMyDiscriminatedUnion : (item: unknown) => item is MyDiscriminatedUnion
+>working : unknown
+>working.type === 'A' : boolean
+>working.type : "A" | "B"
+>working : MyDiscriminatedUnion
+>type : "A" | "B"
+>'A' : "A"
+>working.aProp : number
+>working : { type: "A"; aProp: number; }
+>aProp : number
+
+isMyDiscriminatedUnion(broken) && broken.type === 'A' && broken.aProp;
+>isMyDiscriminatedUnion(broken) && broken.type === 'A' && broken.aProp : number | false
+>isMyDiscriminatedUnion(broken) && broken.type === 'A' : boolean
+>isMyDiscriminatedUnion(broken) : boolean
+>isMyDiscriminatedUnion : (item: unknown) => item is MyDiscriminatedUnion
+>broken : Record<string, any> | undefined
+>broken.type === 'A' : boolean
+>broken.type : "A" | "B"
+>broken : MyDiscriminatedUnion
+>type : "A" | "B"
+>'A' : "A"
+>broken.aProp : number
+>broken : { type: "A"; aProp: number; }
+>aProp : number
+
+isMyDiscriminatedUnion(workingAgain) && workingAgain.type === 'A' && workingAgain.aProp;
+>isMyDiscriminatedUnion(workingAgain) && workingAgain.type === 'A' && workingAgain.aProp : number | false
+>isMyDiscriminatedUnion(workingAgain) && workingAgain.type === 'A' : boolean
+>isMyDiscriminatedUnion(workingAgain) : boolean
+>isMyDiscriminatedUnion : (item: unknown) => item is MyDiscriminatedUnion
+>workingAgain : unknown
+>workingAgain.type === 'A' : boolean
+>workingAgain.type : "A" | "B"
+>workingAgain : MyDiscriminatedUnion
+>type : "A" | "B"
+>'A' : "A"
+>workingAgain.aProp : number
+>workingAgain : { type: "A"; aProp: number; }
+>aProp : number
+

tests/cases/compiler/narrowingUnionToUnion.ts

@@ -212,3 +212,17 @@ function f1x(obj: (string | number)[] | null) {
     assertRelationIsNullOrStringArray(obj);
     obj;  // string[] | null
 }
+
+// Repro from #55425
+
+type MyDiscriminatedUnion = { type: 'A', aProp: number } | { type: 'B', bProp: string };
+
+declare function isMyDiscriminatedUnion(item: unknown): item is MyDiscriminatedUnion;
+
+declare const working: unknown;
+declare const broken: Record<string, any> | undefined;
+declare const workingAgain: Record<string, any> | undefined | unknown;
+
+isMyDiscriminatedUnion(working) && working.type === 'A' && working.aProp;
+isMyDiscriminatedUnion(broken) && broken.type === 'A' && broken.aProp;
+isMyDiscriminatedUnion(workingAgain) && workingAgain.type === 'A' && workingAgain.aProp;