cmd/go: confusing error for non-existent go source files

[rsc]

% go build notexist.go
cannot find package "." in:
	/Users/rsc/go/src/notexist.go
% 

For what it's worth, I'm not in /Users/rsc/go/src.
That's just the error looking for a std package named notexist.go.
It should show me an error more like 'no such file notexist.go'.

/cc @bcmills @jayconrod @matloob

[heschi]

Ping from the release team: any updates on this release blocker?

[jayconrod]

Briefly looked into this, but wasn't able to reproduce it. Outside a module in 1.17.1, I see this:

$ go build notexist.go
no required module provides package notexist.go: go.mod file not found in current directory or any parent directory; see 'go help modules'

Inside a module:

$ go build notexist.go
no required module provides package notexist.go; to add it:
        go get notexist.go

What version of Go was this? What does go env show?

[ianlancetaylor]

@bcmills This is in the 1.18 milestone; time to move to 1.19? Thanks.

(The current error isn't great since it treats nonexist.go as the name of a package. That is technically correct but maybe if it doesn't exist we can treat a trailing ".go" as a special case and give a different error message.)