From 27bec2a0f3959baf1f702981c9817032d646e7dd Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Mateusz=20Burzy=C5=84ski?= <mateuszburzynski@gmail.com>
Date: Wed, 4 Jun 2025 16:21:29 +0200
Subject: [PATCH] Port `Improve `unknown` narrowing by negated type predicates`

---
 internal/checker/flow.go                      |  7 +++++--
 .../narrowUnknownByTypePredicate.types        |  4 ++--
 .../narrowUnknownByTypePredicate.types.diff   | 20 -------------------
 3 files changed, 7 insertions(+), 24 deletions(-)
 delete mode 100644 testdata/baselines/reference/submodule/compiler/narrowUnknownByTypePredicate.types.diff

diff --git a/internal/checker/flow.go b/internal/checker/flow.go
index 4ef3469f75..25a28bcca2 100644
--- a/internal/checker/flow.go
+++ b/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
diff --git a/testdata/baselines/reference/submodule/compiler/narrowUnknownByTypePredicate.types b/testdata/baselines/reference/submodule/compiler/narrowUnknownByTypePredicate.types
index d9f250bdb6..26d83a9a43 100644
--- a/testdata/baselines/reference/submodule/compiler/narrowUnknownByTypePredicate.types
+++ b/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; }
diff --git a/testdata/baselines/reference/submodule/compiler/narrowUnknownByTypePredicate.types.diff b/testdata/baselines/reference/submodule/compiler/narrowUnknownByTypePredicate.types.diff
deleted file mode 100644
index 5425029a9a..0000000000
--- a/testdata/baselines/reference/submodule/compiler/narrowUnknownByTypePredicate.types.diff
+++ /dev/null
@@ -1,20 +0,0 @@
---- old.narrowUnknownByTypePredicate.types
-+++ new.narrowUnknownByTypePredicate.types
-@@= skipped -30, +30 lines =@@
- >value2 : unknown
-
-   value2;
-->value2 : null | undefined
-+>value2 : unknown
- }
-
- declare const value3: unknown;
-@@= skipped -25, +25 lines =@@
- >value4 : unknown
-
-   value4;
-->value4 : {}
-+>value4 : unknown
- }
-
- declare class A { foo: string; }
\ No newline at end of file