Use bidirectional comparability (aka comparability) in narrowing

src/compiler/checker.ts

@@ -25096,7 +25096,7 @@ namespace ts {
                 const narrowedPropType = narrowType(propType);
                 return filterType(type, t => {
                     const discriminantType = getTypeOfPropertyOrIndexSignature(t, propName);
-                    return !(narrowedPropType.flags & TypeFlags.Never) && isTypeComparableTo(narrowedPropType, discriminantType);
+                    return !(narrowedPropType.flags & TypeFlags.Never) && areTypesComparable(narrowedPropType, discriminantType);
                 });
             }
 

tests/baselines/reference/50527.js

@@ -0,0 +1,19 @@
+//// [50527.ts]
+type S = 
+| { type: 'string', value: string } 
+| { type: 'number', value: number } 
+| { type: 'unknown', value: unknown }
+| { value: undefined };
+
+declare var s: S
+
+if (s.value !== undefined) {
+  s;
+}
+
+
+//// [50527.js]
+"use strict";
+if (s.value !== undefined) {
+    s;
+}

tests/baselines/reference/50527.symbols

@@ -0,0 +1,33 @@
+=== tests/cases/compiler/50527.ts ===
+type S = 
+>S : Symbol(S, Decl(50527.ts, 0, 0))
+
+| { type: 'string', value: string } 
+>type : Symbol(type, Decl(50527.ts, 1, 3))
+>value : Symbol(value, Decl(50527.ts, 1, 19))
+
+| { type: 'number', value: number } 
+>type : Symbol(type, Decl(50527.ts, 2, 3))
+>value : Symbol(value, Decl(50527.ts, 2, 19))
+
+| { type: 'unknown', value: unknown }
+>type : Symbol(type, Decl(50527.ts, 3, 3))
+>value : Symbol(value, Decl(50527.ts, 3, 20))
+
+| { value: undefined };
+>value : Symbol(value, Decl(50527.ts, 4, 3))
+
+declare var s: S
+>s : Symbol(s, Decl(50527.ts, 6, 11))
+>S : Symbol(S, Decl(50527.ts, 0, 0))
+
+if (s.value !== undefined) {
+>s.value : Symbol(value, Decl(50527.ts, 1, 19), Decl(50527.ts, 2, 19), Decl(50527.ts, 3, 20), Decl(50527.ts, 4, 3))
+>s : Symbol(s, Decl(50527.ts, 6, 11))
+>value : Symbol(value, Decl(50527.ts, 1, 19), Decl(50527.ts, 2, 19), Decl(50527.ts, 3, 20), Decl(50527.ts, 4, 3))
+>undefined : Symbol(undefined)
+
+  s;
+>s : Symbol(s, Decl(50527.ts, 6, 11))
+}
+

tests/baselines/reference/50527.types

@@ -0,0 +1,33 @@
+=== tests/cases/compiler/50527.ts ===
+type S = 
+>S : { type: 'string'; value: string; } | { type: 'number'; value: number; } | { type: 'unknown'; value: unknown; } | { value: undefined; }
+
+| { type: 'string', value: string } 
+>type : "string"
+>value : string
+
+| { type: 'number', value: number } 
+>type : "number"
+>value : number
+
+| { type: 'unknown', value: unknown }
+>type : "unknown"
+>value : unknown
+
+| { value: undefined };
+>value : undefined
+
+declare var s: S
+>s : S
+
+if (s.value !== undefined) {
+>s.value !== undefined : boolean
+>s.value : unknown
+>s : S
+>value : unknown
+>undefined : undefined
+
+  s;
+>s : { type: "string"; value: string; } | { type: "number"; value: number; } | { type: "unknown"; value: unknown; }
+}
+

tests/cases/compiler/50527.ts

@@ -0,0 +1,13 @@
+// @strict: true
+
+type S = 
+| { type: 'string', value: string } 
+| { type: 'number', value: number } 
+| { type: 'unknown', value: unknown }
+| { value: undefined };
+
+declare var s: S
+
+if (s.value !== undefined) {
+  s;
+}