From 1ef6b24d1359da855fbc2eff6f9b7dbbabd5cb1b Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Mateusz=20Burzy=C5=84ski?= <mateuszburzynski@gmail.com>
Date: Fri, 28 Apr 2023 12:02:13 +0200
Subject: [PATCH] Avoid subtype reduction when creating a union result in
 `discriminateTypeByDiscriminableItems`

---
 src/compiler/checker.ts                       |  2 +-
 .../contextualTypeSelfReferencing.symbols     | 40 +++++++++++++++++++
 .../contextualTypeSelfReferencing.types       | 26 ++++++++++++
 .../compiler/contextualTypeSelfReferencing.ts | 16 ++++++++
 4 files changed, 83 insertions(+), 1 deletion(-)
 create mode 100644 tests/baselines/reference/contextualTypeSelfReferencing.symbols
 create mode 100644 tests/baselines/reference/contextualTypeSelfReferencing.types
 create mode 100644 tests/cases/compiler/contextualTypeSelfReferencing.ts

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 1266af4184020..e474ef533bff8 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -22684,7 +22684,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
                 }
             }
         }
-        const filtered = contains(include, Ternary.False) ? getUnionType(types.filter((_, i) => include[i])) : target;
+        const filtered = contains(include, Ternary.False) ? getUnionType(types.filter((_, i) => include[i]), UnionReduction.None) : target;
         return filtered.flags & TypeFlags.Never ? target : filtered;
     }
 
diff --git a/tests/baselines/reference/contextualTypeSelfReferencing.symbols b/tests/baselines/reference/contextualTypeSelfReferencing.symbols
new file mode 100644
index 0000000000000..3841325def3a2
--- /dev/null
+++ b/tests/baselines/reference/contextualTypeSelfReferencing.symbols
@@ -0,0 +1,40 @@
+=== tests/cases/compiler/contextualTypeSelfReferencing.ts ===
+// repro from https://github.com/microsoft/TypeScript/issues/54048
+
+type narrow<def> = def extends string
+>narrow : Symbol(narrow, Decl(contextualTypeSelfReferencing.ts, 0, 0))
+>def : Symbol(def, Decl(contextualTypeSelfReferencing.ts, 2, 12))
+>def : Symbol(def, Decl(contextualTypeSelfReferencing.ts, 2, 12))
+
+  ? def
+>def : Symbol(def, Decl(contextualTypeSelfReferencing.ts, 2, 12))
+
+  : def extends [unknown, ...unknown[]]
+>def : Symbol(def, Decl(contextualTypeSelfReferencing.ts, 2, 12))
+
+  ? def
+>def : Symbol(def, Decl(contextualTypeSelfReferencing.ts, 2, 12))
+
+  : {
+      [k in keyof def]: narrow<def[k]>;
+>k : Symbol(k, Decl(contextualTypeSelfReferencing.ts, 7, 7))
+>def : Symbol(def, Decl(contextualTypeSelfReferencing.ts, 2, 12))
+>narrow : Symbol(narrow, Decl(contextualTypeSelfReferencing.ts, 0, 0))
+>def : Symbol(def, Decl(contextualTypeSelfReferencing.ts, 2, 12))
+>k : Symbol(k, Decl(contextualTypeSelfReferencing.ts, 7, 7))
+
+    };
+
+declare const parse: <def>(def: narrow<def>) => def;
+>parse : Symbol(parse, Decl(contextualTypeSelfReferencing.ts, 10, 13))
+>def : Symbol(def, Decl(contextualTypeSelfReferencing.ts, 10, 22), Decl(contextualTypeSelfReferencing.ts, 10, 27))
+>def : Symbol(def, Decl(contextualTypeSelfReferencing.ts, 10, 22), Decl(contextualTypeSelfReferencing.ts, 10, 27))
+>narrow : Symbol(narrow, Decl(contextualTypeSelfReferencing.ts, 0, 0))
+>def : Symbol(def, Decl(contextualTypeSelfReferencing.ts, 10, 22), Decl(contextualTypeSelfReferencing.ts, 10, 27))
+>def : Symbol(def, Decl(contextualTypeSelfReferencing.ts, 10, 22), Decl(contextualTypeSelfReferencing.ts, 10, 27))
+
+const result = parse([{ a: "foo" }]);
+>result : Symbol(result, Decl(contextualTypeSelfReferencing.ts, 12, 5))
+>parse : Symbol(parse, Decl(contextualTypeSelfReferencing.ts, 10, 13))
+>a : Symbol(a, Decl(contextualTypeSelfReferencing.ts, 12, 23))
+
diff --git a/tests/baselines/reference/contextualTypeSelfReferencing.types b/tests/baselines/reference/contextualTypeSelfReferencing.types
new file mode 100644
index 0000000000000..2c67613e1e7ca
--- /dev/null
+++ b/tests/baselines/reference/contextualTypeSelfReferencing.types
@@ -0,0 +1,26 @@
+=== tests/cases/compiler/contextualTypeSelfReferencing.ts ===
+// repro from https://github.com/microsoft/TypeScript/issues/54048
+
+type narrow<def> = def extends string
+>narrow : narrow<def>
+
+  ? def
+  : def extends [unknown, ...unknown[]]
+  ? def
+  : {
+      [k in keyof def]: narrow<def[k]>;
+    };
+
+declare const parse: <def>(def: narrow<def>) => def;
+>parse : <def>(def: narrow<def>) => def
+>def : narrow<def>
+
+const result = parse([{ a: "foo" }]);
+>result : [{ a: "foo"; }]
+>parse([{ a: "foo" }]) : [{ a: "foo"; }]
+>parse : <def>(def: narrow<def>) => def
+>[{ a: "foo" }] : [{ a: "foo"; }]
+>{ a: "foo" } : { a: "foo"; }
+>a : "foo"
+>"foo" : "foo"
+
diff --git a/tests/cases/compiler/contextualTypeSelfReferencing.ts b/tests/cases/compiler/contextualTypeSelfReferencing.ts
new file mode 100644
index 0000000000000..f20d5aa1eb26b
--- /dev/null
+++ b/tests/cases/compiler/contextualTypeSelfReferencing.ts
@@ -0,0 +1,16 @@
+// @strict: true
+// @noEmit: true
+
+// repro from https://github.com/microsoft/TypeScript/issues/54048
+
+type narrow<def> = def extends string
+  ? def
+  : def extends [unknown, ...unknown[]]
+  ? def
+  : {
+      [k in keyof def]: narrow<def[k]>;
+    };
+
+declare const parse: <def>(def: narrow<def>) => def;
+
+const result = parse([{ a: "foo" }]);
\ No newline at end of file