Duplicate of #39655:

Quote:

Alternatively, you need to write (_ Result(T)) or ((Result(T))). The reason is that (Result(T)) is interpreted as (Result (T)), i.e., the result is named Result and is of parenthesized type (T), for backwards-compatibility.

https://go.googlesource.com/proposal/+/refs/heads/master/design/go2draft-type-parameters.md#using-generic-types-as-unnamed-function-parameter-types

Also Duplicate of #39656

PS Because of how many people run into this, can there be at least one of them open to investigate if the compiler error message can be improved in all does cases? I understand if it isn't possible and maybe go vet can help here?

There were two syntaxes that I came across that did not work. The second seems covered by #39656, but the first seems like a bug, since this fails (but only for closures):

f := func(b bool) Box(string) { return Box(string){strconv.FormatBool(b)} }