feat: add gochecksumtype linter

[alecthomas]

gochecksumtype is a linter that checks that type switches on "sum types" in Go are exhaustive.

https://github.com/alecthomas/go-check-sumtype

This is based on BurntSushi/go-sumtype, but fixes, modernises and simplifies it.

fixes #402

[CLAassistant]

All committers have signed the CLA.

[ldez]

In order for a pull request adding a linter to be reviewed, the linter and the PR must follow some requirements.

Pull Request Description

• It must have a link to the linter repository.
• It must provide a short description of the linter.

Linter

• It must not be a duplicate of another linter or a rule of a linter. (the team will help to verify that)
• It must have a valid license (AGPL is not allowed) and the file must contain the required information by the license, ex: author, year, etc.
• The linter repository must have a CI and tests.
• It must use go/analysis.
• It must have a valid tag, ex: v1.0.0, v0.1.0.
• It must not contain init().
• It must not contain panic(), log.fatal(), os.exit(), or similar.
• It must not have false positives/negatives. (the team will help to verify that)
• It must have tests inside golangci-lint.

The Linter Tests Inside Golangci-lint

• They must have at least one std lib import.
• They must work with T=<name of the linter test file>.go make test_linters. (the team will help to verify that)

.golangci.reference.yml

• The linter must be added to the list of available linters (alphabetical case-insensitive order).
  • enable and disable options
• If the linter has a configuration, the exhaustive configuration of the linter must be added (alphabetical case-insensitive order)
  • The values must be different from the default ones.
  • The default values must be defined in a comment.
  • The option must have a short description.

Others Requirements

• The files (tests and linter) inside golangci-lint must have the same name as the linter.
• The .golangci.yml of golangci-lint itself must not be edited and the linter must not be added to this file.
• The linters must be sorted in the alphabetical order (case-insensitive) in the Manager.GetAllSupportedLinterConfigs(...) and .golangci.reference.yml.
• The load mode (WithLoadMode(...)):
  • if the linter doesn't use types: goanalysis.LoadModeSyntax
  • goanalysis.LoadModeTypesInfo required WithLoadForGoAnalysis() in the Manager.GetAllSupportedLinterConfigs(...)
• The version in WithSince(...) must be the next minor version (v1.X.0) of golangci-lint.

Recommendations

• The linter should not use SSA. (currently, SSA does not support generics)
• The linter repository should have a readme and linting.
• The linter should be published as a binary. (useful to diagnose bug origins)

---

The golangci-lint team will edit this comment to check the boxes before and during the review.

If the author of the PR is a member of the golangci-lint team, he should not edit this message.

This checklist does not imply that we will accept the linter.

[ldez]

Hello,

this linter seems really close to exhaustive maybe it can be an improvement of this lnter.
ping @nishanths

[alecthomas]

As I mentioned, this is just a fork of an existing linter that has been around for quite some time. There was only a minimal amount of work required to bring it up to date.

FWIW I did look at exhaustive and while it is similar it does not have this functionality. Given the choice between simply updating go-sumtype, or reimplementing all of the logic in exhaustive, I chose the former. I can understand if that's not the right choice for golangci-lint, but I personally don't have the time to reimplement this existing functional lexer in an entirely new codebase. If that's the path forward you'd prefer, I'll close this PR.

[ldez]

I will wait for feedback from @nishanths before taking a decision.

[alecthomas]

Ping!

[ldez]

still waiting for @nishanths feedback

[ldez]

I think we have waited enough time, so we will accept your linter.

@alecthomas Can you take a look at the checklist? #3671 (comment)

[ldez]

Since I added one sts import in the test case, the test fails:

$ T=gochecksumtype.go make test_linters         
GL_TEST_RUN=1 go test -v ./test -count 1 -run TestSourcesFromTestdata/gochecksumtype.go
=== RUN   TestSourcesFromTestdata
    linters_test.go:21: testdata/*.go
=== RUN   TestSourcesFromTestdata/gochecksumtype.go
=== PAUSE TestSourcesFromTestdata/gochecksumtype.go
=== CONT  TestSourcesFromTestdata/gochecksumtype.go
level=info msg="[test] ran [/home/ldez/sources/go/src/github.com/golangci/golangci-lint/golangci-lint run --internal-cmd-test --no-config --allow-parallel-runners --disable-all --out-format=json --max-same-issues=100 -Egochecksumtype gochecksumtype.go] in 213.428115ms"
    analysis.go:66: gochecksumtype.go:26: no diagnostic was reported matching `exhaustiveness check failed for sum type.*SumType.*missing cases for Two`
level=info msg="[test] ran [/home/ldez/sources/go/src/github.com/golangci/golangci-lint/golangci-lint run --internal-cmd-test --no-config --allow-parallel-runners --disable-all --out-format=json --max-same-issues=100 -Etypecheck -Egochecksumtype gochecksumtype.go] in 240.006321ms"
    analysis.go:66: gochecksumtype.go:26: no diagnostic was reported matching `exhaustiveness check failed for sum type.*SumType.*missing cases for Two`
--- FAIL: TestSourcesFromTestdata (2.04s)
    --- FAIL: TestSourcesFromTestdata/gochecksumtype.go (0.45s)
=== RUN   TestSourcesFromTestdataSubDir
--- PASS: TestSourcesFromTestdataSubDir (0.00s)
FAIL
FAIL    github.com/golangci/golangci-lint/test  2.528s
FAIL
make: *** [Makefile:48: test_linters] Error 1

[ldez]

I move the logger and the test works, maybe there is something that I don't understand.

[gnuletik]

Hi ! Thanks for this PR ! I was looking for a similar feature! Is there anything blocking here?

[ldez]

If you can explain why this change produces a different behavior.

[gnuletik]

@ldez thanks for the fast feedback!

It seems that this behavior is intended by the linter's author.

https://github.com/alecthomas/go-check-sumtype/blob/803c965876eec8ffc4d0f26a65af8fa4d2520423/check.go#L64-L65

This behavior comes from upstream:

https://github.com/alecthomas/go-check-sumtype/blob/803c965876eec8ffc4d0f26a65af8fa4d2520423/check.go#L64-L65

NB: The comment is not exact because it seems that the linter only check if the default case is only containing a panic.

[maratori]

This implementation doesn't allow declaring a sum type in package A and checking a type switch in package B. That's because each package is checked separately.

[Antonboom]

maybe to open separate issue?
I feel like this comment will get lost

[maratori]

#4158