Improve detection of cases where subtype reduction is unnecessary

src/compiler/checker.ts

@@ -25864,7 +25864,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
     }
 
     function isTypeSubsetOf(source: Type, target: Type) {
-        return source === target || target.flags & TypeFlags.Union && isTypeSubsetOfUnion(source, target as UnionType);
+        return !!(source === target || source.flags & TypeFlags.Never || target.flags & TypeFlags.Union && isTypeSubsetOfUnion(source, target as UnionType));
     }
 
     function isTypeSubsetOfUnion(source: Type, target: UnionType) {
@@ -26688,7 +26688,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
                 // If an antecedent type is not a subset of the declared type, we need to perform
                 // subtype reduction. This happens when a "foreign" type is injected into the control
                 // flow using the instanceof operator or a user defined type predicate.
-                if (!isTypeSubsetOf(type, declaredType)) {
+                if (!isTypeSubsetOf(type, initialType)) {
                     subtypeReduction = true;
                 }
                 if (isIncomplete(flowType)) {
@@ -26706,7 +26706,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
                         return type;
                     }
                     antecedentTypes.push(type);
-                    if (!isTypeSubsetOf(type, declaredType)) {
+                    if (!isTypeSubsetOf(type, initialType)) {
                         subtypeReduction = true;
                     }
                     if (isIncomplete(flowType)) {
@@ -26781,7 +26781,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
                 // If an antecedent type is not a subset of the declared type, we need to perform
                 // subtype reduction. This happens when a "foreign" type is injected into the control
                 // flow using the instanceof operator or a user defined type predicate.
-                if (!isTypeSubsetOf(type, declaredType)) {
+                if (!isTypeSubsetOf(type, initialType)) {
                     subtypeReduction = true;
                 }
                 // If the type at a particular antecedent path is the declared type there is no

tests/baselines/reference/noSubtypeReduction.symbols

@@ -0,0 +1,51 @@
+=== tests/cases/compiler/noSubtypeReduction.ts ===
+// Repro from #53425
+
+export interface IA {
+>IA : Symbol(IA, Decl(noSubtypeReduction.ts, 0, 0))
+
+    arr: { A: number; }[];
+>arr : Symbol(IA.arr, Decl(noSubtypeReduction.ts, 2, 21))
+>A : Symbol(A, Decl(noSubtypeReduction.ts, 3, 10))
+}
+
+export interface IAB {
+>IAB : Symbol(IAB, Decl(noSubtypeReduction.ts, 4, 1))
+
+    arr: { A: number; B: number; }[];
+>arr : Symbol(IAB.arr, Decl(noSubtypeReduction.ts, 6, 22))
+>A : Symbol(A, Decl(noSubtypeReduction.ts, 7, 10))
+>B : Symbol(B, Decl(noSubtypeReduction.ts, 7, 21))
+}
+
+export function F(x: IA | IAB) {
+>F : Symbol(F, Decl(noSubtypeReduction.ts, 8, 1))
+>x : Symbol(x, Decl(noSubtypeReduction.ts, 10, 18))
+>IA : Symbol(IA, Decl(noSubtypeReduction.ts, 0, 0))
+>IAB : Symbol(IAB, Decl(noSubtypeReduction.ts, 4, 1))
+
+    const useB = (t: number) => { };
+>useB : Symbol(useB, Decl(noSubtypeReduction.ts, 11, 9))
+>t : Symbol(t, Decl(noSubtypeReduction.ts, 11, 18))
+
+    for (const el of x.arr) {
+>el : Symbol(el, Decl(noSubtypeReduction.ts, 12, 14))
+>x.arr : Symbol(arr, Decl(noSubtypeReduction.ts, 2, 21), Decl(noSubtypeReduction.ts, 6, 22))
+>x : Symbol(x, Decl(noSubtypeReduction.ts, 10, 18))
+>arr : Symbol(arr, Decl(noSubtypeReduction.ts, 2, 21), Decl(noSubtypeReduction.ts, 6, 22))
+
+        if ('A' in el) { }
+>el : Symbol(el, Decl(noSubtypeReduction.ts, 12, 14))
+
+        if ('B' in el) {
+>el : Symbol(el, Decl(noSubtypeReduction.ts, 12, 14))
+
+            useB(el.B);
+>useB : Symbol(useB, Decl(noSubtypeReduction.ts, 11, 9))
+>el.B : Symbol(B, Decl(noSubtypeReduction.ts, 7, 21))
+>el : Symbol(el, Decl(noSubtypeReduction.ts, 12, 14))
+>B : Symbol(B, Decl(noSubtypeReduction.ts, 7, 21))
+        }
+    }
+}
+

tests/baselines/reference/noSubtypeReduction.types

@@ -0,0 +1,51 @@
+=== tests/cases/compiler/noSubtypeReduction.ts ===
+// Repro from #53425
+
+export interface IA {
+    arr: { A: number; }[];
+>arr : { A: number; }[]
+>A : number
+}
+
+export interface IAB {
+    arr: { A: number; B: number; }[];
+>arr : { A: number; B: number; }[]
+>A : number
+>B : number
+}
+
+export function F(x: IA | IAB) {
+>F : (x: IA | IAB) => void
+>x : IA | IAB
+
+    const useB = (t: number) => { };
+>useB : (t: number) => void
+>(t: number) => { } : (t: number) => void
+>t : number
+
+    for (const el of x.arr) {
+>el : { A: number; } | { A: number; B: number; }
+>x.arr : { A: number; }[] | { A: number; B: number; }[]
+>x : IA | IAB
+>arr : { A: number; }[] | { A: number; B: number; }[]
+
+        if ('A' in el) { }
+>'A' in el : boolean
+>'A' : "A"
+>el : { A: number; } | { A: number; B: number; }
+
+        if ('B' in el) {
+>'B' in el : boolean
+>'B' : "B"
+>el : { A: number; } | { A: number; B: number; }
+
+            useB(el.B);
+>useB(el.B) : void
+>useB : (t: number) => void
+>el.B : number
+>el : { A: number; B: number; }
+>B : number
+        }
+    }
+}
+

tests/cases/compiler/noSubtypeReduction.ts

@@ -0,0 +1,22 @@
+// @strict: true
+// @noEmit: true
+
+// Repro from #53425
+
+export interface IA {
+    arr: { A: number; }[];
+}
+
+export interface IAB {
+    arr: { A: number; B: number; }[];
+}
+
+export function F(x: IA | IAB) {
+    const useB = (t: number) => { };
+    for (const el of x.arr) {
+        if ('A' in el) { }
+        if ('B' in el) {
+            useB(el.B);
+        }
+    }
+}