Avoid subtype reduction when creating a union result in discriminateTypeByDiscriminableItems

src/compiler/checker.ts

@@ -22712,7 +22712,7 @@ 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;
     }
 

tests/baselines/reference/contextualTypeSelfReferencing.symbols

@@ -0,0 +1,42 @@
+//// [tests/cases/compiler/contextualTypeSelfReferencing.ts] ////
+
+=== 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))
+

tests/baselines/reference/contextualTypeSelfReferencing.types

@@ -0,0 +1,28 @@
+//// [tests/cases/compiler/contextualTypeSelfReferencing.ts] ////
+
+=== 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"
+

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" }]);