Fix "Union types are not being narrowed down correctly in 4.3 #44401" (#44771)

* Fix "Union types are not being narrowed down correctly in 4.3 #44401"

* On undefined constraint, add the original type to constraints.

src/compiler/checker.ts

@@ -18363,7 +18363,7 @@ namespace ts {
                     // parameter 'T extends 1 | 2', the intersection 'T & 1' should be reduced to '1' such that it doesn't
                     // appear to be comparable to '2'.
                     if (relation === comparableRelation && target.flags & TypeFlags.Primitive) {
-                        const constraints = sameMap((source as IntersectionType).types, t => t.flags & TypeFlags.Primitive ? t : getBaseConstraintOfType(t) || unknownType);
+                        const constraints = sameMap((source as IntersectionType).types, getBaseConstraintOrType);
                         if (constraints !== (source as IntersectionType).types) {
                             source = getIntersectionType(constraints);
                             if (!(source.flags & TypeFlags.Intersection)) {

tests/baselines/reference/intersectionOfUnionNarrowing.js

@@ -0,0 +1,26 @@
+//// [intersectionOfUnionNarrowing.ts]
+interface X {
+  a?: { aProp: string };
+  b?: { bProp: string };
+}
+type AorB = { a: object; b: undefined } | { a: undefined; b: object };
+
+declare const q: X & AorB;
+
+if (q.a !== undefined) {
+  q.a.aProp;
+} else {
+  // q.b is previously incorrectly inferred as potentially undefined
+  q.b.bProp;
+}
+
+
+//// [intersectionOfUnionNarrowing.js]
+"use strict";
+if (q.a !== undefined) {
+    q.a.aProp;
+}
+else {
+    // q.b is previously incorrectly inferred as potentially undefined
+    q.b.bProp;
+}

tests/baselines/reference/intersectionOfUnionNarrowing.symbols

@@ -0,0 +1,47 @@
+=== tests/cases/conformance/types/intersection/intersectionOfUnionNarrowing.ts ===
+interface X {
+>X : Symbol(X, Decl(intersectionOfUnionNarrowing.ts, 0, 0))
+
+  a?: { aProp: string };
+>a : Symbol(X.a, Decl(intersectionOfUnionNarrowing.ts, 0, 13))
+>aProp : Symbol(aProp, Decl(intersectionOfUnionNarrowing.ts, 1, 7))
+
+  b?: { bProp: string };
+>b : Symbol(X.b, Decl(intersectionOfUnionNarrowing.ts, 1, 24))
+>bProp : Symbol(bProp, Decl(intersectionOfUnionNarrowing.ts, 2, 7))
+}
+type AorB = { a: object; b: undefined } | { a: undefined; b: object };
+>AorB : Symbol(AorB, Decl(intersectionOfUnionNarrowing.ts, 3, 1))
+>a : Symbol(a, Decl(intersectionOfUnionNarrowing.ts, 4, 13))
+>b : Symbol(b, Decl(intersectionOfUnionNarrowing.ts, 4, 24))
+>a : Symbol(a, Decl(intersectionOfUnionNarrowing.ts, 4, 43))
+>b : Symbol(b, Decl(intersectionOfUnionNarrowing.ts, 4, 57))
+
+declare const q: X & AorB;
+>q : Symbol(q, Decl(intersectionOfUnionNarrowing.ts, 6, 13))
+>X : Symbol(X, Decl(intersectionOfUnionNarrowing.ts, 0, 0))
+>AorB : Symbol(AorB, Decl(intersectionOfUnionNarrowing.ts, 3, 1))
+
+if (q.a !== undefined) {
+>q.a : Symbol(a, Decl(intersectionOfUnionNarrowing.ts, 0, 13), Decl(intersectionOfUnionNarrowing.ts, 4, 13), Decl(intersectionOfUnionNarrowing.ts, 0, 13), Decl(intersectionOfUnionNarrowing.ts, 4, 43))
+>q : Symbol(q, Decl(intersectionOfUnionNarrowing.ts, 6, 13))
+>a : Symbol(a, Decl(intersectionOfUnionNarrowing.ts, 0, 13), Decl(intersectionOfUnionNarrowing.ts, 4, 13), Decl(intersectionOfUnionNarrowing.ts, 0, 13), Decl(intersectionOfUnionNarrowing.ts, 4, 43))
+>undefined : Symbol(undefined)
+
+  q.a.aProp;
+>q.a.aProp : Symbol(aProp, Decl(intersectionOfUnionNarrowing.ts, 1, 7))
+>q.a : Symbol(a, Decl(intersectionOfUnionNarrowing.ts, 0, 13), Decl(intersectionOfUnionNarrowing.ts, 4, 13))
+>q : Symbol(q, Decl(intersectionOfUnionNarrowing.ts, 6, 13))
+>a : Symbol(a, Decl(intersectionOfUnionNarrowing.ts, 0, 13), Decl(intersectionOfUnionNarrowing.ts, 4, 13))
+>aProp : Symbol(aProp, Decl(intersectionOfUnionNarrowing.ts, 1, 7))
+
+} else {
+  // q.b is previously incorrectly inferred as potentially undefined
+  q.b.bProp;
+>q.b.bProp : Symbol(bProp, Decl(intersectionOfUnionNarrowing.ts, 2, 7))
+>q.b : Symbol(b, Decl(intersectionOfUnionNarrowing.ts, 1, 24), Decl(intersectionOfUnionNarrowing.ts, 4, 57))
+>q : Symbol(q, Decl(intersectionOfUnionNarrowing.ts, 6, 13))
+>b : Symbol(b, Decl(intersectionOfUnionNarrowing.ts, 1, 24), Decl(intersectionOfUnionNarrowing.ts, 4, 57))
+>bProp : Symbol(bProp, Decl(intersectionOfUnionNarrowing.ts, 2, 7))
+}
+

tests/baselines/reference/intersectionOfUnionNarrowing.types

@@ -0,0 +1,44 @@
+=== tests/cases/conformance/types/intersection/intersectionOfUnionNarrowing.ts ===
+interface X {
+  a?: { aProp: string };
+>a : { aProp: string; } | undefined
+>aProp : string
+
+  b?: { bProp: string };
+>b : { bProp: string; } | undefined
+>bProp : string
+}
+type AorB = { a: object; b: undefined } | { a: undefined; b: object };
+>AorB : AorB
+>a : object
+>b : undefined
+>a : undefined
+>b : object
+
+declare const q: X & AorB;
+>q : X & AorB
+
+if (q.a !== undefined) {
+>q.a !== undefined : boolean
+>q.a : ({ aProp: string; } & object) | undefined
+>q : X & AorB
+>a : ({ aProp: string; } & object) | undefined
+>undefined : undefined
+
+  q.a.aProp;
+>q.a.aProp : string
+>q.a : { aProp: string; } & object
+>q : X & { a: object; b: undefined; }
+>a : { aProp: string; } & object
+>aProp : string
+
+} else {
+  // q.b is previously incorrectly inferred as potentially undefined
+  q.b.bProp;
+>q.b.bProp : string
+>q.b : { bProp: string; } & object
+>q : X & { a: undefined; b: object; }
+>b : { bProp: string; } & object
+>bProp : string
+}
+

tests/cases/conformance/types/intersection/intersectionOfUnionNarrowing.ts

@@ -0,0 +1,16 @@
+// @strict: true
+
+interface X {
+  a?: { aProp: string };
+  b?: { bProp: string };
+}
+type AorB = { a: object; b: undefined } | { a: undefined; b: object };
+
+declare const q: X & AorB;
+
+if (q.a !== undefined) {
+  q.a.aProp;
+} else {
+  // q.b is previously incorrectly inferred as potentially undefined
+  q.b.bProp;
+}