Type containing templated, branded string is not assignable to itself

[HansBrende]

🔎 Search Terms

brand branded tag tagged template string not assignable extends different type name unrelated

🕗 Version & Regression Information

• This changed between versions 5.0.4 and 5.1.6

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/FAUwHgDg9gTgLgAgDYkQQwFwIBQB4AqAfNgJQIC8hC+C4cIAdgCYDOCABgCQDeA5GrwQAyBNwSYEARgQBfGewQB+KQiwAmMuQQMArkiQBCANyhIsRCkQAjLHiKkKVGnUasOPfoJFiJ0uQuVpdU1tPUMTYDQKBCsjIA

💻 Code

export let a: (<T>() => T extends `${'a' & { a: 1 }}` ? 1 : 2) = null!;
export let b: (<T>() => T extends `${'a' & { a: 1 }}` ? 1 : 2) = null!;

a = b; // ❌ Error!

🙁 Actual behavior

Typescript error:

Type
'<T>() => T extends `${"a" & { a: 1; }}` ? 1 : 2' is not assignable to type
'<T>() => T extends `${"a" & { a: 1; }}` ? 1 : 2'.
Two different types with this name exist, but they are unrelated.

🙂 Expected behavior

No error

Additional information about the issue

I would expect that, as a general principle, any type which is copy-and-pasted from one place to another should be assignable to itself. (With the exception of unique symbol, since by design it's supposed to not do what I just said.) This is perhaps the most lax notion of assignability that could possibly exist, I would think.

Note that, as soon as I remove either the outer template or the inner brand, assignability works as expected once again.

[HansBrende]

Note: bisecting the issue reveals that the regression was introduced in #52836.

[HansBrende]

Update: appears this issue is also present for the string mapping types like Uppercase, not just template strings. E.g.:

export let a: <T>() => T extends Uppercase<'a' & { a: 1 }> ? 1 : 2 = null!;
export let b: <T>() => T extends Uppercase<'a' & { a: 1 }> ? 1 : 2 = null!;

a = b; // ❌ Error!

[HansBrende]

I believe this issue can be fixed by inserting the following code at line 23006 of checker.ts, in the function structuredTypeRelatedToWorker:

if (sourceFlags & TypeFlags.TemplateLiteral) {
    if (arrayIsEqualTo((source as TemplateLiteralType).texts, (target as TemplateLiteralType).texts)) {
        const sourceTypes = (source as TemplateLiteralType).types;
        const targetTypes = (target as TemplateLiteralType).types;
        result = Ternary.True;
        for (let i = 0; i < sourceTypes.length; i++) {
            if (!(result &= isRelatedTo(sourceTypes[i], targetTypes[i], RecursionFlags.Both, false))) {
                break;
            }
        }
        return result;
    }
}
if (sourceFlags & TypeFlags.StringMapping) {
    if ((source as StringMappingType).symbol === (target as StringMappingType).symbol) {
        return isRelatedTo((source as IndexType).type, (target as IndexType).type, RecursionFlags.Both, false);
    }
}

Let me know if you want me to open a PR.

[RyanCavanaugh]

We can look at a PR, but we won't merge it if it re-regresses #52345

[HansBrende]

@RyanCavanaugh cool, PR submitted! Not sure how to test if the fix re-regesses #52345 however.