go/types, types2: improved constant handling guarded by go1.21 but other inference improvements are not

[griesemer]

The Go 1.21 compiler implements significantly more powerful and precise type inference than Go 1.20. One of the improvements made is for untyped constants (#58671). That change was implemented based on the language version: -lang must be >= go1.21.

func f[P any](...P) {}

func main() {
	f(1, 2.0)
}

produces an error (default type float64 of 2.0 does not match inferred type int for P) with -lang=go1.20 but runs fine with -lang >= go1.21.

However, there are plenty of other inference improvements that that were made for 1.21 which are accessible even if -lang < go1.21. For instance:

package main

func f[T any](interface{ m(T) }) {}

type S struct{}

func (S) m(int) {}

func main() {
	f(S{})
}

In this case the change and error is not fundamentally different from above: in Go 1.20 we couldn't infer a type argument and now we can. In both these cases one can provide explicit type arguments and make the code compile. Also, all inference improvements are fully backwards-compatible.

I propose that we remove the -lang check for the more lenient constant handling for consistency with the other inference improvements. (Making all improvements -lang dependent is not practical.)

[griesemer]

cc: @findleyr @ianlancetaylor for feedback

[gopherbot]

Change https://go.dev/cl/499997 mentions this issue: go/types, types2: remove version check for more lenient constant handling in inference

[ianlancetaylor]

I am OK with this.

[findleyr]

I am fine with this too. If it proves to be a problem, we can always put back the -lang check: making -lang more compatible with the released Go version should always be OK.