Have getAssignmentReducedType use the comparable relation instead of typeMaybeAssignableTo.

src/compiler/checker.ts

@@ -13791,18 +13791,6 @@ namespace ts {
             return flow.id;
         }
 
-        function typeMaybeAssignableTo(source: Type, target: Type) {
-            if (!(source.flags & TypeFlags.Union)) {
-                return isTypeAssignableTo(source, target);
-            }
-            for (const t of (<UnionType>source).types) {
-                if (isTypeAssignableTo(t, target)) {
-                    return true;
-                }
-            }
-            return false;
-        }
-
         // Remove those constituent types of declaredType to which no constituent type of assignedType is assignable.
         // For example, when a variable of type number | string | boolean is assigned a value of type number | boolean,
         // we remove type string.
@@ -13811,7 +13799,7 @@ namespace ts {
                 if (assignedType.flags & TypeFlags.Never) {
                     return assignedType;
                 }
-                const reducedType = filterType(declaredType, t => typeMaybeAssignableTo(assignedType, t));
+                const reducedType = filterType(declaredType, t => isTypeComparableTo(assignedType, t));
                 if (!(reducedType.flags & TypeFlags.Never)) {
                     return reducedType;
                 }

tests/baselines/reference/assignmentGenericLookupTypeNarrowing.js

@@ -0,0 +1,23 @@
+//// [assignmentGenericLookupTypeNarrowing.ts]
+// Repro from #26130
+
+let mappedObject: {[K in "foo"]: null | {x: string}} = {foo: {x: "hello"}};
+declare function foo<T>(x: T): null | T;
+
+function bar<K extends "foo">(key: K) {
+  const element = foo(mappedObject[key]);
+  if (element == null)
+    return;
+  const x = element.x;
+}
+
+
+//// [assignmentGenericLookupTypeNarrowing.js]
+// Repro from #26130
+var mappedObject = { foo: { x: "hello" } };
+function bar(key) {
+    var element = foo(mappedObject[key]);
+    if (element == null)
+        return;
+    var x = element.x;
+}

tests/baselines/reference/assignmentGenericLookupTypeNarrowing.symbols

@@ -0,0 +1,40 @@
+=== tests/cases/conformance/expressions/assignmentOperator/assignmentGenericLookupTypeNarrowing.ts ===
+// Repro from #26130
+
+let mappedObject: {[K in "foo"]: null | {x: string}} = {foo: {x: "hello"}};
+>mappedObject : Symbol(mappedObject, Decl(assignmentGenericLookupTypeNarrowing.ts, 2, 3))
+>K : Symbol(K, Decl(assignmentGenericLookupTypeNarrowing.ts, 2, 20))
+>x : Symbol(x, Decl(assignmentGenericLookupTypeNarrowing.ts, 2, 41))
+>foo : Symbol(foo, Decl(assignmentGenericLookupTypeNarrowing.ts, 2, 56))
+>x : Symbol(x, Decl(assignmentGenericLookupTypeNarrowing.ts, 2, 62))
+
+declare function foo<T>(x: T): null | T;
+>foo : Symbol(foo, Decl(assignmentGenericLookupTypeNarrowing.ts, 2, 75))
+>T : Symbol(T, Decl(assignmentGenericLookupTypeNarrowing.ts, 3, 21))
+>x : Symbol(x, Decl(assignmentGenericLookupTypeNarrowing.ts, 3, 24))
+>T : Symbol(T, Decl(assignmentGenericLookupTypeNarrowing.ts, 3, 21))
+>T : Symbol(T, Decl(assignmentGenericLookupTypeNarrowing.ts, 3, 21))
+
+function bar<K extends "foo">(key: K) {
+>bar : Symbol(bar, Decl(assignmentGenericLookupTypeNarrowing.ts, 3, 40))
+>K : Symbol(K, Decl(assignmentGenericLookupTypeNarrowing.ts, 5, 13))
+>key : Symbol(key, Decl(assignmentGenericLookupTypeNarrowing.ts, 5, 30))
+>K : Symbol(K, Decl(assignmentGenericLookupTypeNarrowing.ts, 5, 13))
+
+  const element = foo(mappedObject[key]);
+>element : Symbol(element, Decl(assignmentGenericLookupTypeNarrowing.ts, 6, 7))
+>foo : Symbol(foo, Decl(assignmentGenericLookupTypeNarrowing.ts, 2, 75))
+>mappedObject : Symbol(mappedObject, Decl(assignmentGenericLookupTypeNarrowing.ts, 2, 3))
+>key : Symbol(key, Decl(assignmentGenericLookupTypeNarrowing.ts, 5, 30))
+
+  if (element == null)
+>element : Symbol(element, Decl(assignmentGenericLookupTypeNarrowing.ts, 6, 7))
+
+    return;
+  const x = element.x;
+>x : Symbol(x, Decl(assignmentGenericLookupTypeNarrowing.ts, 9, 7))
+>element.x : Symbol(x, Decl(assignmentGenericLookupTypeNarrowing.ts, 2, 41))
+>element : Symbol(element, Decl(assignmentGenericLookupTypeNarrowing.ts, 6, 7))
+>x : Symbol(x, Decl(assignmentGenericLookupTypeNarrowing.ts, 2, 41))
+}
+

tests/baselines/reference/assignmentGenericLookupTypeNarrowing.types

@@ -0,0 +1,43 @@
+=== tests/cases/conformance/expressions/assignmentOperator/assignmentGenericLookupTypeNarrowing.ts ===
+// Repro from #26130
+
+let mappedObject: {[K in "foo"]: null | {x: string}} = {foo: {x: "hello"}};
+>mappedObject : { foo: { x: string; }; }
+>null : null
+>x : string
+>{foo: {x: "hello"}} : { foo: { x: string; }; }
+>foo : { x: string; }
+>{x: "hello"} : { x: string; }
+>x : string
+>"hello" : "hello"
+
+declare function foo<T>(x: T): null | T;
+>foo : <T>(x: T) => T
+>x : T
+>null : null
+
+function bar<K extends "foo">(key: K) {
+>bar : <K extends "foo">(key: K) => void
+>key : K
+
+  const element = foo(mappedObject[key]);
+>element : { foo: { x: string; }; }[K]
+>foo(mappedObject[key]) : { foo: { x: string; }; }[K]
+>foo : <T>(x: T) => T
+>mappedObject[key] : { foo: { x: string; }; }[K]
+>mappedObject : { foo: { x: string; }; }
+>key : K
+
+  if (element == null)
+>element == null : boolean
+>element : { foo: { x: string; }; }[K]
+>null : null
+
+    return;
+  const x = element.x;
+>x : string
+>element.x : string
+>element : { foo: { x: string; }; }[K]
+>x : string
+}
+

tests/baselines/reference/enumAssignmentCompat3.types

@@ -252,9 +252,9 @@ abc = merged; // missing 'd'
 >merged : Merged.E
 
 merged = abc; // ok
->merged = abc : First.E
+>merged = abc : First.E.a | First.E.b
 >merged : Merged.E
->abc : First.E
+>abc : First.E.a | First.E.b
 
 abc = merged2; // ok
 >abc = merged2 : Merged2.E

tests/cases/conformance/expressions/assignmentOperator/assignmentGenericLookupTypeNarrowing.ts

@@ -0,0 +1,11 @@
+// Repro from #26130
+
+let mappedObject: {[K in "foo"]: null | {x: string}} = {foo: {x: "hello"}};
+declare function foo<T>(x: T): null | T;
+
+function bar<K extends "foo">(key: K) {
+  const element = foo(mappedObject[key]);
+  if (element == null)
+    return;
+  const x = element.x;
+}