Properly handle tagged primitives in control flow analysis

src/compiler/checker.ts

@@ -22013,7 +22013,10 @@ namespace ts {
                 return reduceLeft((<UnionType>type).types, (facts, t) => facts | getTypeFacts(t), TypeFacts.None);
             }
             if (flags & TypeFlags.Intersection) {
-                return reduceLeft((<UnionType>type).types, (facts, t) => facts & getTypeFacts(t), TypeFacts.All);
+                // When an intersection contains a primitive type we ignore object type constituents as they are
+                // presumably type tags. For example, in string & { __kind__: "name" } we ignore the object type.
+                const containsPrimitive = maybeTypeOfKind(type, TypeFlags.Primitive);
+                return reduceLeft((<UnionType>type).types, (facts, t) => t.flags & TypeFlags.Object && containsPrimitive ? facts : facts & getTypeFacts(t), TypeFacts.All);
             }
             return TypeFacts.All;
         }

tests/baselines/reference/taggedPrimitiveNarrowing.js

@@ -0,0 +1,26 @@
+//// [taggedPrimitiveNarrowing.ts]
+type Hash = string & { __hash: true };
+
+function getHashLength(hash: Hash): number {
+    if (typeof hash !== "string") {
+        throw new Error("This doesn't look like a hash");
+    }
+    return hash.length;
+}
+
+
+//// [taggedPrimitiveNarrowing.js]
+"use strict";
+function getHashLength(hash) {
+    if (typeof hash !== "string") {
+        throw new Error("This doesn't look like a hash");
+    }
+    return hash.length;
+}
+
+
+//// [taggedPrimitiveNarrowing.d.ts]
+declare type Hash = string & {
+    __hash: true;
+};
+declare function getHashLength(hash: Hash): number;

tests/baselines/reference/taggedPrimitiveNarrowing.symbols

@@ -0,0 +1,22 @@
+=== tests/cases/compiler/taggedPrimitiveNarrowing.ts ===
+type Hash = string & { __hash: true };
+>Hash : Symbol(Hash, Decl(taggedPrimitiveNarrowing.ts, 0, 0))
+>__hash : Symbol(__hash, Decl(taggedPrimitiveNarrowing.ts, 0, 22))
+
+function getHashLength(hash: Hash): number {
+>getHashLength : Symbol(getHashLength, Decl(taggedPrimitiveNarrowing.ts, 0, 38))
+>hash : Symbol(hash, Decl(taggedPrimitiveNarrowing.ts, 2, 23))
+>Hash : Symbol(Hash, Decl(taggedPrimitiveNarrowing.ts, 0, 0))
+
+    if (typeof hash !== "string") {
+>hash : Symbol(hash, Decl(taggedPrimitiveNarrowing.ts, 2, 23))
+
+        throw new Error("This doesn't look like a hash");
+>Error : Symbol(Error, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+    }
+    return hash.length;
+>hash.length : Symbol(String.length, Decl(lib.es5.d.ts, --, --))
+>hash : Symbol(hash, Decl(taggedPrimitiveNarrowing.ts, 2, 23))
+>length : Symbol(String.length, Decl(lib.es5.d.ts, --, --))
+}
+

tests/baselines/reference/taggedPrimitiveNarrowing.types

@@ -0,0 +1,27 @@
+=== tests/cases/compiler/taggedPrimitiveNarrowing.ts ===
+type Hash = string & { __hash: true };
+>Hash : Hash
+>__hash : true
+>true : true
+
+function getHashLength(hash: Hash): number {
+>getHashLength : (hash: Hash) => number
+>hash : Hash
+
+    if (typeof hash !== "string") {
+>typeof hash !== "string" : boolean
+>typeof hash : "string" | "number" | "bigint" | "boolean" | "symbol" | "undefined" | "object" | "function"
+>hash : Hash
+>"string" : "string"
+
+        throw new Error("This doesn't look like a hash");
+>new Error("This doesn't look like a hash") : Error
+>Error : ErrorConstructor
+>"This doesn't look like a hash" : "This doesn't look like a hash"
+    }
+    return hash.length;
+>hash.length : number
+>hash : Hash
+>length : number
+}
+

tests/cases/compiler/taggedPrimitiveNarrowing.ts

@@ -0,0 +1,11 @@
+// @strict: true
+// @declaration: true
+
+type Hash = string & { __hash: true };
+
+function getHashLength(hash: Hash): number {
+    if (typeof hash !== "string") {
+        throw new Error("This doesn't look like a hash");
+    }
+    return hash.length;
+}