any | T from conditional type does not reduce to any

[dragomirtitian]

Bug Report

🔎 Search Terms

any union

🕗 Version & Regression Information

• This changed between versions 5.0.0-dev.20221207 and 5.0.0-dev.20221208

⏯ Playground Link

Playground link with relevant code

💻 Code

type Spec = any extends object ? any : string;

type WithSpec<T extends number> = T

type R = WithSpec<Spec> // error here  in 5.0

🙁 Actual behavior

Spec is any | string so it can't be used with WithSpec

🙂 Expected behavior

any | string is reduced to any as it was in 4.9

[Tobias-Scholz]

I believe this is caused by #50397 and #51621.

In 4.9 we had a check in getSubstitutionType where the baseType was returned when both baseType and constraint where non-generic. This check was removed in #51621, so a SubstitutionType is created for any.

A simple fix might be to just return the baseType when the baseType is any.

function getSubstitutionType(baseType: Type, constraint: Type) {
  if (constraint.flags & TypeFlags.AnyOrUnknown || constraint === baseType || baseType.flags & TypeFlags.Any) {
    return baseType;
  }