cmd/go2go: A type alias that references a parameterized type in another package causes a panic

[firelizzard18]

If package x contains a type X that is a parameterized type, and package y contains a type Y = x.X, go tool go2go build will fail with a panic upon attempting to instantiate type Y.

This fails because spec *ast.TypeSpec for an alias has spec.TParams == nil, which causes a panic in *translator.instantiateTypeDecl. A workaround is to modify *translator.findTypeSpec to resolve type aliases to the *ast.TypeSpec of the underlying type:

diff --git a/src/go/go2go/instantiate.go b/src/go/go2go/instantiate.go
index bcb38de64a..2351ee6c13 100644
--- a/src/go/go2go/instantiate.go
+++ b/src/go/go2go/instantiate.go
@@ -232,8 +232,18 @@ func (t *translator) findTypeSpec(qid qualifiedIdent) (*ast.TypeSpec, error) {
                return nil, fmt.Errorf("could not find Object for %q", qid)
        }
        spec, ok := t.importer.lookupTypeSpec(obj)
-       if !ok {
-               return nil, fmt.Errorf("could not find type spec for %q", qid)
+       for {
+               if !ok {
+                       return nil, fmt.Errorf("could not find type spec for %q", qid)
+               }
+               if spec.TParams != nil {
+                       break
+               }
+               named, ok := t.importer.info.TypeOf(spec.Type).(*types.Named)
+               if !ok || named.Obj().IsAlias() {
+                       break
+               }
+               spec, ok = t.importer.lookupTypeSpec(named.Obj())
        }
        return spec, nil
 }

What version of Go are you using (go version)?

$ go version
go version devel +2faeebf4e5 Tue Jun 23 04:54:48 2020 +0000 linux/amd64

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GOARCH="amd64"
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"

What did you do?

• git clone https://gitlab.com/firelizzard/go-iter -b packages
• cd go-iter
• go tool go2go build

What did you expect to see?

A successful build

What did you see instead?

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x8 pc=0x60f161]

goroutine 1 [running]:
go/go2go.(*translator).instantiateTypeDecl(0xc0004f5698, 0x0, 0xc000181620, 0xc000112f30, 0xc000181680, 0x2, 0x2, 0xc000369020, 0x2, 0x2, ...)
        /usr/lib/go-tip/src/go/go2go/instantiate.go:157 +0x81
go/go2go.(*translator).translateTypeInstantiation(0xc0004f5698, 0xc000182380)
        /usr/lib/go-tip/src/go/go2go/rewrite.go:909 +0x4e8
go/go2go.(*translator).translateExpr(0xc0004f5698, 0xc000182380)
        /usr/lib/go-tip/src/go/go2go/rewrite.go:657 +0x385
go/go2go.(*translator).translateExpr(0xc0004f5698, 0xc000177bf0)
        /usr/lib/go-tip/src/go/go2go/rewrite.go:660 +0x365
go/go2go.(*translator).translateExprList(0xc0004f5698, 0xc000177bf0, 0x1, 0x1)
        /usr/lib/go-tip/src/go/go2go/rewrite.go:791 +0x46
go/go2go.(*translator).translateExpr(0xc0004f5698, 0xc000177c00)
        /usr/lib/go-tip/src/go/go2go/rewrite.go:659 +0x349
go/go2go.(*translator).translateExprList(0xc0004f5698, 0xc000177c00, 0x1, 0x1)
        /usr/lib/go-tip/src/go/go2go/rewrite.go:791 +0x46
go/go2go.(*translator).translateStmt(0xc0004f5698, 0xc00012e940)
        /usr/lib/go-tip/src/go/go2go/rewrite.go:559 +0x29d
go/go2go.(*translator).translateBlockStmt(0xc0004f5698, 0xc00017f5c0)
        /usr/lib/go-tip/src/go/go2go/rewrite.go:520 +0x57
go/go2go.(*translator).translateFuncDecl(0xc0004f5698, 0xc00011c970)
        /usr/lib/go-tip/src/go/go2go/rewrite.go:510 +0xc5
go/go2go.(*translator).translate(0xc0004f5698, 0xc00011ca00)
        /usr/lib/go-tip/src/go/go2go/rewrite.go:437 +0x391
go/go2go.rewriteAST(0xc00010a240, 0xc000024420, 0x0, 0x0, 0xc0001847d0, 0xc00011ca00, 0xc0005a2400, 0x0, 0x0)
        /usr/lib/go-tip/src/go/go2go/rewrite.go:241 +0xe5
go/go2go.rewriteFile(0x693853, 0x1, 0xc00010a240, 0xc000024420, 0x0, 0x0, 0xc0001847d0, 0xc000013130, 0xd, 0xc00011ca00, ...)
        /usr/lib/go-tip/src/go/go2go/rewrite.go:205 +0xb8
go/go2go.rewriteFilesInPath(0xc000024420, 0x0, 0x0, 0x693853, 0x1, 0xc000132280, 0x6, 0x13, 0x0, 0x0, ...)
        /usr/lib/go-tip/src/go/go2go/go2go.go:104 +0xc11
go/go2go.rewriteToPkgs(0xc000024420, 0x0, 0x0, 0x693853, 0x1, 0xc00000c200, 0xc0000748d0, 0xc0000748a0, 0xc000074870, 0xc000074840)
        /usr/lib/go-tip/src/go/go2go/go2go.go:46 +0x165
go/go2go.Rewrite(...)
        /usr/lib/go-tip/src/go/go2go/go2go.go:30
main.translate(0xc000024420, 0x693853, 0x1)
        /usr/lib/go-tip/src/cmd/go2go/translate.go:15 +0x47
main.main()
        /usr/lib/go-tip/src/cmd/go2go/main.go:68 +0x9da

[gopherbot]

Change https://golang.org/cl/239598 mentions this issue: [dev.go2go] go/go2go: resolve type alias of parameterized type

[ianlancetaylor]

Thanks for pointing that out. I wasn't able to recreate the problem using the go-iter package, but I was able to create a small test case based on your description. The problem shown by that test case is now fixed on the dev.go2go branch.