While I have experience extracting string from string[], this particular scenario is proving to be quite challenging:
type example<T = boolean> = true;
// with just "example", how can I retrieve the template parameter "boolean" in this case?
type T = example extends example<infer U> ? U : never;
I recall that achieving this was deemed impossible a few years back, but it's worth exploring whether any advancements have been made since then.
The main issue at hand is the fact that the generic type parameter in this scenario is not being utilized:
type Foo<T = number> = 123;
type Oops = Foo extends Foo<infer U> ? U : never;
// type Oops = unknown
In this case, Foo<T> will always be 123 regardless of the value of T. TypeScript's type system relies on a structural rather than nominal approach. Therefore, without a structural dependency on T, inferring T from Foo<T> will not be reliable, even if it occasionally works. This is why the TypeScript FAQ discourages the use of generic types that do not utilize their type parameters.
Another significant issue is that TypeScript does not consider default type arguments for inference purposes, as discussed in microsoft/TypeScript#42064.
Even if TypeScript treated types nominally, the desired behavior may not be achieved. Contrast this with using constraints, which play a partial role in inference:
type Bar<T extends number = number> = 123;
type Hmm = Bar extends Bar<infer U> ? U : never;
// type Hmm = number
Applying the same logic to Foo<T> would suggest that Bar<T> would also encounter issues, but here you obtain number instead of
unknown</code because the compiler resorts to the constraint when inference fails. Despite <code>Bar<T>123 for all T, there is still a constraint of number associated with T.
Combining both factors indicates that the intended action is unattainable. To rectify this, the approach should involve utilizing the type parameter explicitly and explicitly stating the type without a type argument to prompt the compiler to substitute in the default value, thereby increasing the likelihood of accurately inferring the type.
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