go/types: var declaration changes representation of its type (alias related) #66704
Description
This is a minimized repro of another gotypesalias=1 problem affecting the x/tools gcimporter.
The root cause is clearly in go/types (at b24ec88).
package types // "go/types"
func TestAliasProblem(t *testing.T) {
t.Setenv("GODEBUG", "gotypesalias=1")
const src = `package a
var x T[B] // <-- this causes the problem! deleting it causes type of B to be ok
type T[_ any] struct{}
type A T[B]
type B = T[A]
`
fset := token.NewFileSet()
f, err := parser.ParseFile(fset, "a.go", src, 0)
if err != nil {
t.Fatal(err)
}
pkg, err := new(Config).Check("a", fset, []*ast.File{f}, nil)
if err != nil {
t.Fatal(err)
}
B := pkg.Scope().Lookup("B")
t.Error(Unalias(B.Type())) // "invalid type" if var x is present, "a.T[a.A]" otherwise
}