cmd/compile: prevent typecheck importer reading type parameter twice

This is a port of CL 349009 to typecheck importer.

Fixes #48306

Change-Id: Iec3f078089346bd85f0ab739896e079940325011
Reviewed-on: https://go-review.googlesource.com/c/go/+/349011
Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dan Scales <danscales@google.com>

src/cmd/compile/internal/typecheck/iimport.go

@@ -316,16 +316,12 @@ func (r *importReader) doDecl(sym *types.Sym) *ir.Name {
 		return n
 
 	case 'T', 'U':
-		var rparams []*types.Type
-		if tag == 'U' {
-			rparams = r.typeList()
-		}
-
 		// Types can be recursive. We need to setup a stub
 		// declaration before recursing.
 		n := importtype(pos, sym)
 		t := n.Type()
 		if tag == 'U' {
+			rparams := r.typeList()
 			t.SetRParams(rparams)
 		}
 

src/go/internal/gcimporter/gcimporter_test.go

@@ -167,8 +167,6 @@ func TestImportTypeparamTests(t *testing.T) {
 	skip := map[string]string{
 		"equal.go":  "inconsistent embedded sorting", // TODO(rfindley): investigate this.
 		"nested.go": "fails to compile",              // TODO(rfindley): investigate this.
-
-		"issue46461.go": "known issue with type parameter constraints referring back to parameterized type",
 	}
 
 	for _, entry := range list {

test/run.go

@@ -2188,8 +2188,6 @@ var g3Failures = setOf(
 
 	"typeparam/nested.go", // -G=3 doesn't support function-local types with generics
 
-	"typeparam/issue46461b.go", // -G=3 fails when type parameters refer back to the parameterized type itself
-
 	"typeparam/mdempsky/4.go", // -G=3 can't export functions with labeled breaks in loops
 )
 

test/typeparam/issue48306.dir/a.go

@@ -0,0 +1,9 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package a
+
+type I[T I[T]] interface {
+	F() T
+}

test/typeparam/issue48306.dir/main.go

@@ -0,0 +1,15 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import "a"
+
+type S struct{}
+
+func (*S) F() *S { return nil }
+
+func main() {
+	var _ a.I[*S] = &S{}
+}

test/typeparam/issue48306.go

@@ -0,0 +1,7 @@
+// rundir -G=3
+
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ignored