[release-branch.go1.18] cmd/compile/internal/importer: key tparams by Package instead of pkgname

The importer type param index used package name as type parameter key,
causing type parameters to be reused/overwritten if two packages in the
import graph had the same combination of (package name, declaration
name, type parameter name).

Fix this by instead using the *Package in the key.

Note: -G=3 was added to typeparam/issue51836.go, as it is necessary for
1.18 but not for tip.

For #51836
Fixes #51847

Change-Id: I881ceaf3cf7c1ab4e0835962350feb552e79b233
Reviewed-on: https://go-review.googlesource.com/c/go/+/394219
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
(cherry picked from commit fd1b5904ae)
Reviewed-on: https://go-review.googlesource.com/c/go/+/394854

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

@@ -53,7 +53,7 @@ const (
 )
 
 type ident struct {
-	pkg  string
+	pkg  *types2.Package
 	name string
 }
 
@@ -402,7 +402,7 @@ func (r *importReader) obj(name string) {
 		t := types2.NewTypeParam(tn, nil)
 		// To handle recursive references to the typeparam within its
 		// bound, save the partial type in tparamIndex before reading the bounds.
-		id := ident{r.currPkg.Name(), name}
+		id := ident{r.currPkg, name}
 		r.p.tparamIndex[id] = t
 
 		var implicit bool
@@ -687,7 +687,7 @@ func (r *importReader) doType(base *types2.Named) types2.Type {
 			errorf("unexpected type param type")
 		}
 		pkg, name := r.qualifiedIdent()
-		id := ident{pkg.Name(), name}
+		id := ident{pkg, name}
 		if t, ok := r.p.tparamIndex[id]; ok {
 			// We're already in the process of importing this typeparam.
 			return t

src/go/internal/gcimporter/iimport.go

@@ -53,7 +53,7 @@ const (
 )
 
 type ident struct {
-	pkg  string
+	pkg  *types.Package
 	name string
 }
 
@@ -393,7 +393,7 @@ func (r *importReader) obj(name string) {
 		t := types.NewTypeParam(tn, nil)
 		// To handle recursive references to the typeparam within its
 		// bound, save the partial type in tparamIndex before reading the bounds.
-		id := ident{r.currPkg.Name(), name}
+		id := ident{r.currPkg, name}
 		r.p.tparamIndex[id] = t
 
 		var implicit bool
@@ -676,7 +676,7 @@ func (r *importReader) doType(base *types.Named) types.Type {
 			errorf("unexpected type param type")
 		}
 		pkg, name := r.qualifiedIdent()
-		id := ident{pkg.Name(), name}
+		id := ident{pkg, name}
 		if t, ok := r.p.tparamIndex[id]; ok {
 			// We're already in the process of importing this typeparam.
 			return t

test/typeparam/issue51836.dir/a.go

@@ -0,0 +1,8 @@
+// Copyright 2022 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 T[K any] struct {
+}

test/typeparam/issue51836.dir/aa.go

@@ -0,0 +1,13 @@
+// Copyright 2022 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
+
+import (
+	"./a"
+)
+
+type T[K any] struct {
+	t a.T[K]
+}

test/typeparam/issue51836.dir/p.go

@@ -0,0 +1,11 @@
+// Copyright 2022 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 p
+
+import (
+	a "./aa"
+)
+
+var Foo a.T[int]

test/typeparam/issue51836.go

@@ -0,0 +1,7 @@
+// compiledir -s -G=3
+
+// Copyright 2022 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