x/tools/go/packages: Alias type not being parsed as types.Alias with packages.Load

[shashankram]

I am not sure why an aliased type, MyInt below is not being parsed as types.Alias with golang.org/x/tools/go/packages.Load.

In example.com/project/foo/file.go:

package foo

type MyInt = int

var _ = MyInt(1)

In foo_test.go:

import (
        "go/types"

        "golang.org/x/tools/go/packages"
)

func TestLoad(t *testing.T) {
	cfg := packages.Config{
		Mode: packages.NeedName |
			packages.NeedFiles | packages.NeedImports | packages.NeedDeps |
			packages.NeedModule | packages.NeedTypes | packages.NeedSyntax,
		BuildFlags: []string{"-tags", ""},
		Tests:      false,
	}

	pkgs, err := packages.Load(&cfg, "example.com/project/foo")
	if err != nil {
		t.Errorf("unexpected error: %v", err)
	}
	if len(pkgs) != 1 {
		t.Errorf("expected 1 package, got %d", len(pkgs))
	}
	obj := pkgs[0].Types.Scope().Lookup("MyInt")
	if obj == nil {
		t.Errorf("expected type MyInt, got nil")
	}
	typ := obj.Type()
	_, ok := typ.(*types.Alias)
	if !ok {
		t.Errorf("expected type MyInt to be an alias, got %T", typ)
	}
}

Am I missing something?

I am using Go 1.24.2

[mateusz834]

On my machine it works 😄.

[mateusz@arch bb ]$ go test -v .
=== RUN   TestLoad
--- PASS: TestLoad (0.01s)
PASS
ok  	bb	(cached)

I would suggest looking into the go.mod file, possibly you have a go version < 1.23 (in such case the gotypesalias GODEBUG is set to off by default.). Also try running your program with GODEBUG=gotypesalias=1.

[shashankram]

I would suggest looking into the go.mod file, possibly you have a go version < 1.23 (in such case the gotypesalias GODEBUG is set to off by default.). Also try running your program with GODEBUG=gotypesalias=1.

@mateusz834 I think you are right. My go.mod has go 1.20 specified as the min version to import.

When I run the test from the IDE and print runtime.Version(), I see 1.24.2, which I assumed causes gotypesalias to be enabled by default. I guess that is not the case?

=== RUN   TestLoad
=== RUN   TestLoad/type_alias
    /src/code/project/foo/file_test.go:1190: go version: go1.24.2
    /src/code/project/foo/file_test.go:1215: expected type MyInt to be an alias, got *types.Basic
--- FAIL: TestLoad (0.50s)
    --- FAIL: TestLoad/type_alias (0.50s)
FAIL

However, when I run it from the shell:

GODEBUG=gotypesalias=1 go test -race ./foo/ -run TestLoad -count=1
ok  	example.com/project/foo	2.897s

GODEBUG=gotypesalias=0 go test -race ./foo/ -run TestLoad -count=1
--- FAIL: TestLoad (1.60s)
    --- FAIL: TestLoad/type_alias (1.60s)
        file_test.go:1190: go version: go1.24.2
        file_test.go:1215: expected type MyInt to be an alias, got *types.Basic

[seankhliao]

The runtime version and language version are not the same. A newer runtime can implement an older language version, gated by the go.mod go directive.

[mateusz834]

@shashankram See https://go.dev/doc/godebug#default if you are curious how it works.

[shashankram]

@mateusz834 is there a way to access the godebug settings at runtime? E.g., gotypesalias is enabled by default in 1.23, so is there a way I can check its value at runtime when an env is not set but the setting is enabled by default?

[mateusz834]

is there a way to access the godebug settings at runtime? E.g., gotypesalias is enabled by default in 1.23, so is there a way I can check its value at runtime when an env is not set but the setting is enabled by default?

I don't think so, but you can take look at:

https://github.com/golang/tools/blob/3e377036196f644e59e757af8a38ea6afa07677c/internal/aliases/aliases_go122.go#L61-L80