[dev.regabi] go/types: must not import a package called "init"

This is a port of CL 287494 to go/types. The additional checks in
test/fixedbugs are included, though they won't be executed by go/types.
Support for errorcheckdir checks will be added to go/types in a later
CL.

Change-Id: I37e202ea5daf7d7b8fc6ae93a4c4dbd11762480f
Reviewed-on: https://go-review.googlesource.com/c/go/+/290570
Reviewed-by: Robert Griesemer <gri@golang.org>
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>

src/go/types/resolver.go

@@ -252,14 +252,6 @@ func (check *Checker) collectObjects() {
 					return
 				}
 
-				// add package to list of explicit imports
-				// (this functionality is provided as a convenience
-				// for clients; it is not needed for type-checking)
-				if !pkgImports[imp] {
-					pkgImports[imp] = true
-					pkg.imports = append(pkg.imports, imp)
-				}
-
 				// local name overrides imported package name
 				name := imp.name
 				if d.spec.Name != nil {
@@ -269,10 +261,19 @@ func (check *Checker) collectObjects() {
 						check.errorf(d.spec.Name, _ImportCRenamed, `cannot rename import "C"`)
 						return
 					}
-					if name == "init" {
-						check.errorf(d.spec.Name, _InvalidInitDecl, "cannot declare init - must be func")
-						return
-					}
+				}
+
+				if name == "init" {
+					check.errorf(d.spec.Name, _InvalidInitDecl, "cannot import package as init - init must be a func")
+					return
+				}
+
+				// add package to list of explicit imports
+				// (this functionality is provided as a convenience
+				// for clients; it is not needed for type-checking)
+				if !pkgImports[imp] {
+					pkgImports[imp] = true
+					pkg.imports = append(pkg.imports, imp)
 				}
 
 				pkgName := NewPkgName(d.spec.Pos(), pkg, name, imp)

src/go/types/testdata/importdecl0/importdecl0a.src

@@ -10,7 +10,7 @@ import (
 	// we can have multiple blank imports (was bug)
 	_ "math"
 	_ "net/rpc"
-	init /* ERROR "cannot declare init" */ "fmt"
+	init /* ERROR "cannot import package as init" */ "fmt"
 	// reflect defines a type "flag" which shows up in the gc export data
 	"reflect"
 	. /* ERROR "imported but not used" */ "reflect"

test/fixedbugs/issue43962.dir/a.go

@@ -0,0 +1,5 @@
+// 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 init

test/fixedbugs/issue43962.dir/b.go

@@ -0,0 +1,7 @@
+// 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 b
+
+import "./a" // ERROR "cannot import package as init"

test/fixedbugs/issue43962.go

@@ -0,0 +1,9 @@
+// errorcheckdir
+
+// 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.
+
+// Issue 43962: Importing a package called "init" is an error.
+
+package ignored