The first error looks exactly right. It's saying you declared T twice, at line 13 (19 and 33 are the column numbers).
The second error is kind of spurious, resulting from the fact that the thing you're calling didn't typecheck correctly. That error should probably be suppressed.

Perhaps correct code is like this: https://go.dev/play/p/sd9bia4ybSv?v=gotip

@randall77 Does showing full path in "/home/kamil/go/src/github.com/KZiemian/Go-s11-26.go:13:19: other declaration of T" is correct? It feels wrong to me, but this may be just my.

@hopehook I didn't think how correct code would look like. I just tried to test Go1.18beta in every way that I can imagine, even the silly one, and look if something crash/give weird result. I now working through Type Parameters Proposal to find answers to few true problems that I have. This is just byproduct of it.

I see, that sometimes-relative-sometimes-absolute thing is weird.

I can't reproduce by invoking the compiler directly, but I can using go build. It happens with other errors like this also.

package main

func f(x int, x int) {
}

% ~/go1.18beta2/bin/go tool compile tmp1.go
tmp1.go:3:15: x redeclared in this block
	tmp1.go:3:8: other declaration of x
% ~/go1.18beta2/bin/go build tmp1.go       
# command-line-arguments
./tmp1.go:3:15: x redeclared in this block
	/Users/khr/gowork/tmp1.go:3:8: other declaration of x

Change https://go.dev/cl/385755 mentions this issue: cmd/go: enable file shortening for lines starting with \t