Improve comparable relation of template literal types to detect more impossible situations

🔎 Search Terms

string condition always return false
2367 template

🕗 Version & Regression Information

This is the behavior in every version after 4.4.4 (including nightly), and I reviewed the FAQ for entries about "template" or "string"

This issue claims to fix a related problem in 4.5, but 4.5.5 is the first issue that fails for me. Maybe no relation though.
https://github.com/microsoft/TypeScript/issues/45201

In 4.4.4 it it fails correctly though, so maybe that fix is the regression of this issue?! https://www.typescriptlang.org/play?ts=4.4.4#code/C4TwDgpgBAglC8UAGBDAXGgJAbwHYFcBbAIwgCcBfJAKFEigCEFliMcCTyrrbxoANZkgAebPEVKUadaAE0h7CVxrUAxgHtcAZ2BR0sIeizYAjNw3bdrRkNbGzKizqiiogxCLEO1m5yDRQ8h443tQAlgBmUAAUwgjwiCAAlFDY1FBQTuoANhAAdNnqAObRAOTAABZhWlBaFer42QAmUBUoAG7QEShhuU2lSdQUPJExKPGIxClpGVm5BcVl3b01GmRkEKrA2SADQ9RAA

⏯ Playground Link

https://www.typescriptlang.org/play?#code/C4TwDgpgBAglC8UAGBDAXGgJAbwHYFcBbAIwgCcBfJAKFEigCEFliMcCTyrrbxoANZkgAebPEVKUadaAE0h7CVxrUAxgHtcAZ2BR0sIeizYAjNw3bdrRkNbGzKizqiiogxCLEO1m5yDRQ8h443tQAlgBmUAAUwgjwiCAAlFDY1FBQTuoANhAAdNnqAObRAOTAABZhWlBaFer42QAmUBUoAG7QEShhuU2lSdQUPJExKPGIxClpGVm5BcVl3b01GmRkEKrA2SADQ9RAA

💻 Code

type A = `a::${number}`
type B = `b::${number}`

type X = `x::${number}`
type Y = `${number}`

const a: A = `a::${1}`
const b: B = `b::${1}`

const x: X = `x::${1}`
const y: Y = `${1}`

if (x === y) {
  console.log('this should have failed')
}

if (a === b) {
  console.log('fails correctly')
}

🙁 Actual behavior

Comparing x and y generates no complaints from TS for "x::1" and "1", but does for "a::1" and "b::1". There is no way for a number to evaluate to "x:: so there is no overlap between these template types.

🙂 Expected behavior

I would expect that Typescript shows that error consistently whenever types don't match.

There is no overlap between a string that must start with "x::" and one that must start with a number.

Additional information about the issue

No response

Start by reproducing the comparison in the linked TypeScript Playground and read the related issue 45201 for context. Trace how the type checker compares the template literal types; done means it reports the missing no-overlap diagnostic for x and y without regressing the existing a and b behavior.