cmd/go: 'go mod edit' can add requirements that it can't remove

[bcmills]

go mod edit -require can be used to add versions that don't match the major version of the corresponding module.

Unfortunately, the major-version validation happens early during parsing, so go mod edit -droprequire cannot subsequently drop those requirements.

example.com$ gotip mod init example.com
go: creating new go.mod: module example.com

example.com$ gotip version
go version devel +9838f4df Fri Jun 14 18:41:07 2019 +0000 linux/amd64

example.com$ gotip mod edit -require golang.org/x/text@v2.0.0

example.com$ gotip mod edit -droprequire golang.org/x/text
go: errors parsing go.mod:
/tmp/tmp.5UPKq2KvZc/example.com/go.mod:5: invalid module: golang.org/x/text should be v0 or v1, not v2 (v2.0.0)

example.com$ cat go.mod
module example.com

go 1.13

require golang.org/x/text v2.0.0

example.com$

CC @jayconrod

See also #30513, #26454.

[hyangah]

FYI #27060 asked to validate the added requirement but was closed as working as intended.

[bcmills]

Yeah, I think the fix for this should go in the other direction: we should move the major-version validation from modfile.Parse to some later step.

[gopherbot]

Change https://go.dev/cl/671575 mentions this issue: cmd/go: allow editing of invalid go.mod

[iwdgo]

modfile.Parse() returns a nil module for any error.
modfile.ParseLax() does the same although the documentation states

ParseLax is like Parse but ignores unknown statements

Where unknown statements would extend to incorrect or wrongly written known statements.
On tip, go mod edit is unusable on a go.mod if parsing returns an error, some examples are:

• https://go.dev/play/p/6WZvRK28hgi?v=gotip (cmd/go: get downgrade of excluded last version fails #26454)
• https://go.dev/play/p/Orc8fhhny7B?v=gotip (cmd/go: mod edit -fmt reports latest as invalid version #28439)
• https://go.dev/play/p/GAH7tXQLl3F?v=gotip (cmd/go: 'go mod edit' can add requirements that it can't remove #32630)

In the same way, go mod edit -print of go mod edit -json are also unusable when go.mod is erroneous.
The proposal is to return the parsed go.mod even when there is an error from ParseLax used with nil fix argument.
Returning the errors unfortunately fails make.bash.

The submitted CL illustrates the proposal. It requires to modify x/mod/modfile.