cmd/compile: no need to infer type arguments for blank type parameters with `any` constraint

[griesemer]

package p

func f[_ any]() {}

func _() {
        f()
}

produces the error

cannot infer _

but blank type parameters can't be used and thus it doesn't matter what their corresponding argument is if the constraint is any (any type will satisfy that constraint). If the constraint is not any we should still infer the type argument so we can verify constraint satisfaction.

Maybe we shouldn't report an error in this case. Or maybe this is so esoteric that is doesn't matter. Filing this issue so we document decision (and perhaps implement it).

cc: @ianlancetaylor @findleyr for input

[findleyr]

In my opinion we shouldn't do anything special for this case, i.e. we should continue to report an error. It doesn't really matter that the type parameter is blank here -- all that really matters is that the function type does not depend on the type parameter.
We could equally well support the following:

func f[A, B any](a A) A { return a }

var g = f(0)

Choosing to omit this error for blank type parameters seems somewhat arbitrary. In your proposal, would we support the example here if B were _? If so, that's rather more complex as we'd have to short-circuit inference.

[bcmills]

If we omitted the error for the blank-parameter case, then it would be impossible to change the name of the parameter from _ to any other name — even though that kind of renaming should otherwise be completely compatible.

[griesemer]

The arguments against not inferring (i.e., for not doing anything special) in this case are convincing. Let's not make any changes. Closing.