Fix logic error in ported code

internal/checker/flow.go

@@ -718,8 +718,8 @@ func (c *Checker) narrowTypeByDiscriminant(t *Type, access *ast.Node, narrowType
 	}
 	narrowedPropType := narrowType(propType)
 	return c.filterType(t, func(t *Type) bool {
-		discriminantType := c.getTypeOfPropertyOrIndexSignatureOfType(t, propName)
-		return discriminantType == nil || discriminantType.flags&TypeFlagsNever == 0 && narrowedPropType.flags&TypeFlagsNever == 0 && c.areTypesComparable(narrowedPropType, discriminantType)
+		discriminantType := core.OrElse(c.getTypeOfPropertyOrIndexSignatureOfType(t, propName), c.unknownType)
+		return discriminantType.flags&TypeFlagsNever == 0 && narrowedPropType.flags&TypeFlagsNever == 0 && c.areTypesComparable(narrowedPropType, discriminantType)
 	})
 }
 

testdata/baselines/reference/compiler/switchExhaustiveNarrowing.symbols

@@ -0,0 +1,49 @@
+//// [tests/cases/compiler/switchExhaustiveNarrowing.ts] ////
+
+=== switchExhaustiveNarrowing.ts ===
+interface ClientSource {
+>ClientSource : Symbol(ClientSource, Decl(switchExhaustiveNarrowing.ts, 0, 0))
+
+  type: "client";
+>type : Symbol(type, Decl(switchExhaustiveNarrowing.ts, 0, 24))
+}
+
+interface ServiceSource {
+>ServiceSource : Symbol(ServiceSource, Decl(switchExhaustiveNarrowing.ts, 2, 1))
+
+  type: "service";
+>type : Symbol(type, Decl(switchExhaustiveNarrowing.ts, 4, 25))
+}
+
+function isDisplaySource(source: ClientSource | ServiceSource | undefined): boolean {
+>isDisplaySource : Symbol(isDisplaySource, Decl(switchExhaustiveNarrowing.ts, 6, 1))
+>source : Symbol(source, Decl(switchExhaustiveNarrowing.ts, 8, 25))
+>ClientSource : Symbol(ClientSource, Decl(switchExhaustiveNarrowing.ts, 0, 0))
+>ServiceSource : Symbol(ServiceSource, Decl(switchExhaustiveNarrowing.ts, 2, 1))
+
+  switch (source?.type) {
+>source?.type : Symbol(type, Decl(switchExhaustiveNarrowing.ts, 0, 24), Decl(switchExhaustiveNarrowing.ts, 4, 25))
+>source : Symbol(source, Decl(switchExhaustiveNarrowing.ts, 8, 25))
+>type : Symbol(type, Decl(switchExhaustiveNarrowing.ts, 0, 24), Decl(switchExhaustiveNarrowing.ts, 4, 25))
+
+    case "client":
+      return true;
+    case "service":
+      return false;
+    case undefined:
+>undefined : Symbol(undefined)
+
+      return false;
+    default:
+      neverReached(source);
+>neverReached : Symbol(neverReached, Decl(switchExhaustiveNarrowing.ts, 20, 1))
+>source : Symbol(source, Decl(switchExhaustiveNarrowing.ts, 8, 25))
+
+      return false;
+  }
+}
+
+function neverReached(_v: never): void {}
+>neverReached : Symbol(neverReached, Decl(switchExhaustiveNarrowing.ts, 20, 1))
+>_v : Symbol(_v, Decl(switchExhaustiveNarrowing.ts, 22, 22))
+

testdata/baselines/reference/compiler/switchExhaustiveNarrowing.types

@@ -0,0 +1,55 @@
+//// [tests/cases/compiler/switchExhaustiveNarrowing.ts] ////
+
+=== switchExhaustiveNarrowing.ts ===
+interface ClientSource {
+  type: "client";
+>type : "client"
+}
+
+interface ServiceSource {
+  type: "service";
+>type : "service"
+}
+
+function isDisplaySource(source: ClientSource | ServiceSource | undefined): boolean {
+>isDisplaySource : (source: ClientSource | ServiceSource | undefined) => boolean
+>source : ClientSource | ServiceSource | undefined
+
+  switch (source?.type) {
+>source?.type : "client" | "service" | undefined
+>source : ClientSource | ServiceSource | undefined
+>type : "client" | "service" | undefined
+
+    case "client":
+>"client" : "client"
+
+      return true;
+>true : true
+
+    case "service":
+>"service" : "service"
+
+      return false;
+>false : false
+
+    case undefined:
+>undefined : undefined
+
+      return false;
+>false : false
+
+    default:
+      neverReached(source);
+>neverReached(source) : void
+>neverReached : (_v: never) => void
+>source : never
+
+      return false;
+>false : false
+  }
+}
+
+function neverReached(_v: never): void {}
+>neverReached : (_v: never) => void
+>_v : never
+

testdata/tests/cases/compiler/switchExhaustiveNarrowing.ts

@@ -0,0 +1,26 @@
+// @strict: true
+// @noEmit: true
+
+interface ClientSource {
+  type: "client";
+}
+
+interface ServiceSource {
+  type: "service";
+}
+
+function isDisplaySource(source: ClientSource | ServiceSource | undefined): boolean {
+  switch (source?.type) {
+    case "client":
+      return true;
+    case "service":
+      return false;
+    case undefined:
+      return false;
+    default:
+      neverReached(source);
+      return false;
+  }
+}
+
+function neverReached(_v: never): void {}