Template literal type derived from string is not a subtype of the TLT derived from any

🔎 Search Terms

"template literal type", "any", "string", "mutually assignable types"

🕗 Version & Regression Information

• This changed between versions 4.1.5 and 4.2.3
• This changed in commit or PR 411c6d04c6007cd3bb74ef0ce6cc11e8c699a2b5 (reported by every-ts)

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=4.2.3#code/C4TwDgpgBA4g9nAJgRigXigAwCQG8B2ArgLYBGEATgL6ZQQAewE+iAzlngIb4g1QD8UYBULQAXFABmnADasIAbigB6ZUJEQAUKEiwEiAEzoOuUgEsA5mfzA+DJi3Y5c3XrUHDRUCdLmKVap5aOtDwSADMxs6kCDIQ3HaMzGwmrnweGt5SsvJKquqi2uCh+gAsUXhEMjKJDinOae4F4tl+eYEaRbphiACsFbiELBCS1hCItclOXDzpzVm+uQHNml3QAEKcKAOswtYWk46ps01BCzn++WfWUKUAdMh3vQA0rfJQN-cGd+FrUJuGHZ7fAWKAAHygRDIlHBUHMVhssJicDi3FhVRksKGiBGYwmtHsU2ObgE8x8F3a80+Dyer0W0Gp31+mgAxnB8LsoHAANYSBonYwAcmQgqgnGmBBI5GomAUmnyAHluR98LcaS8oGziGAZJxrOxGT9WezOWZWKxRHyZiSMMLReKTLsKPsaHKgA

💻 Code

type Good1 = `${number}` extends `${any}` ? true : false; // true
type Good2 = `${bigint}` extends `${any}` ? true : false; // true
type Good3 = `${boolean}` extends `${any}` ? true : false; // true
type Good4 = `${null}` extends `${any}` ? true : false; // true
type Good5 = `${undefined}` extends `${any}` ? true : false; // true

type Bad1 = `${string}` extends `${any}` ? true : false; // true in 4.1.5, false in 4.2.3
type Bad2 = `${string | number | bigint | boolean | null | undefined}` extends `${any}` ? true : false; // true in 4.1.5, false in 4.2.3

const ok: `${any}` = '1' as `${number}`;
// Ok in 4.1.5, complains in 4.2.3
const issue: `${any}` = '1' as `${string}`;

🙁 Actual behavior

Type 'string' is not assignable to type '${any}'.(2322)

🙂 Expected behavior

Should compile with no errors, since `${any}` and string should be the same type (or at least mutually assignable).

Additional information about the issue

Proposed solution: simplify the type `${any}` to string like it is done with the `${string}` type.

Rationale: for any type X that is either concrete or generic (with a restriction to valid arguments of a template literal type) the type `${string|X}` simplifies to string. Therefore, if TypeScript strives to any consistency, we would expect `${string|any}` (i.e. `${any}`) to also simplify to string.

I have no idea why we currently have `${any}` extends `${number}` true (which could break with my suggestion if no other changes are made), but this bug report is not concerted with this. I'm leaving this up to the maintainers.

Proposed solution 2: Just make string extends `${any}` and `${any}` extends string true.

Start with the linked TypeScript Playground reproduction and compare the behavior between versions 4.1.5 and 4.2.3. Then inspect commit 411c6d04c6007cd3bb74ef0ce6cc11e8c699a2b5 and the template literal type checker paths it changed. Done means the ${string} and ${any} assignability cases compile consistently with the expected behavior.