Port "Improve unknown narrowing by negated type predicates" (#1044)

Author: Andarist

internal/checker/flow.go

@@ -841,10 +841,13 @@ func (c *Checker) getNarrowedTypeWorker(t *Type, candidate *Type, assumeTrue boo
 				return !c.isTypeDerivedFrom(t, candidate)
 			})
 		}
+		if t.flags&TypeFlagsUnknown != 0 {
+			t = c.unknownUnionType
+		}
 		trueType := c.getNarrowedType(t, candidate, true /*assumeTrue*/, false /*checkDerived*/)
-		return c.filterType(t, func(t *Type) bool {
+		return c.recombineUnknownType(c.filterType(t, func(t *Type) bool {
 			return !c.isTypeSubsetOf(t, trueType)
-		})
+		}))
 	}
 	if t.flags&TypeFlagsAnyOrUnknown != 0 {
 		return candidate

testdata/baselines/reference/submodule/compiler/narrowUnknownByTypePredicate.types

@@ -31,7 +31,7 @@ if (!isNotNullish(value2)) {
 >value2 : unknown
 
   value2;
->value2 : unknown
+>value2 : null | undefined
 }
 
 declare const value3: unknown;
@@ -56,7 +56,7 @@ if (!isNullish(value4)) {
 >value4 : unknown
 
   value4;
->value4 : unknown
+>value4 : {}
 }
 
 declare class A { foo: string; }