Use comparability for discriminant properties when narrowing types for a default switch clause (#2734)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: jakebailey <5341706+jakebailey@users.noreply.github.com>

Author: Copilot

internal/checker/flow.go

@@ -1112,7 +1112,9 @@ func (c *Checker) narrowTypeBySwitchOnDiscriminant(t *Type, data *ast.FlowSwitch
 		if t.flags&TypeFlagsUndefined == 0 {
 			u = c.getRegularTypeOfLiteralType(c.extractUnitType(t))
 		}
-		return !slices.Contains(switchTypes, u)
+		return !slices.ContainsFunc(switchTypes, func(st *Type) bool {
+			return isUnitType(st) && c.areTypesComparable(st, u)
+		})
 	})
 	if caseType.flags&TypeFlagsNever != 0 {
 		return defaultType

testdata/baselines/reference/submodule/conformance/discriminatedUnionTypes4.errors.txt

@@ -186,7 +186,7 @@ function action3(animal: Animal) {
       check(animal);
 >check(animal) : void
 >check : (p: never) => void
->animal : Animal
+>animal : never
   }
 }
 

testdata/baselines/reference/submodule/conformance/discriminatedUnionTypes4.errors.txt.diff

@@ -60,12 +60,4 @@
 +>animal : { type: "dog"; bark: string; }
  >bark : string
 
-       break;
-@@= skipped -8, +8 lines =@@
-       check(animal);
- >check(animal) : void
- >check : (p: never) => void
-->animal : never
-+>animal : Animal
-   }
- }
+       break;