cmd/compile: fix identity case relating to 'any' and shape types

In identical(), we don't want any to match a shape empty-interface type
for the identStrict option, since IdenticalStrict() is specifically not
supposed to match a shape type with a non-shape type.

There is similar code in (*Type).cmp() (TINTER case), but I don't
believe that we want to disqualify shape types from matching any in this
case, since cmp() is used for back-end code, where we don't care about
shape types vs non-shape types.

The issue mainly comes about when 'any' is used as a type argument
(rather than 'interface{}'), but only with some complicated
circumstances, as shown by the test case. (Couldn't reproduce with
simpler test cases.)

Fixes #50109

Change-Id: I3f2f88be158f9ad09273237e1d346bc56aac099f
Reviewed-on: https://go-review.googlesource.com/c/go/+/371154
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

2 files changed, 106 insertions, 0 deletions

diff --git a/test/typeparam/issue50109.go b/test/typeparam/issue50109.go
new file mode 100644
index 0000000000..a6913df843
--- /dev/null
+++ b/test/typeparam/issue50109.go
@@ -0,0 +1,105 @@
+// run -gcflags=-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 main
+
+import (
+	"fmt"
+)
+
+type AnyCacher[T any] interface {
+	// Get an item from the cache. Returns the item or nil, and a bool indicating
+	// whether the key was found.
+	Get(k string) (T, bool)
+	// Add an item to the cache, replacing any existing item.
+	Set(k string, x T)
+}
+
+// Item ...
+type Item[T any] struct {
+	Object T
+}
+
+// AnyCache implements AnyCacher
+type AnyCache[T any] struct {
+	*anyCache[T]
+}
+
+type anyCache[T any] struct {
+	items   map[string]Item[T]
+	janitor *janitor[T] // Needed for the failure in the issue
+}
+
+// Set adds an item to the cache, replacing any existing item.
+func (c *anyCache[T]) Set(k string, x T) {
+	c.items[k] = Item[T]{
+		Object: x,
+	}
+}
+
+// Get gets an item from the cache. Returns the item or nil, and a bool indicating
+// whether the key was found.
+func (c *anyCache[T]) Get(k string) (T, bool) {
+	// "Inlining" of get and Expired
+	item, found := c.items[k]
+	if !found {
+		var ret T
+		return ret, false
+	}
+
+	return item.Object, true
+}
+
+type janitor[T any] struct {
+	stop chan bool
+}
+
+func newAnyCache[T any](m map[string]Item[T]) *anyCache[T] {
+	c := &anyCache[T]{
+		items: m,
+	}
+	return c
+}
+
+// NewAny[T any](...) returns a new AnyCache[T].
+func NewAny[T any]() *AnyCache[T] {
+	items := make(map[string]Item[T])
+	return &AnyCache[T]{newAnyCache(items)}
+}
+
+// NewAnyCacher[T any](...) returns an AnyCacher[T] interface.
+func NewAnyCacher[T any]() AnyCacher[T] {
+	return NewAny[T]()
+}
+
+type MyStruct struct {
+	Name string
+}
+
+func main() {
+	// Create a generic cache.
+	// All items are cached as interface{} so they need to be cast back to their
+	// original type when retrieved.
+	// Failure in issue doesn't happen with 'any' replaced by 'interface{}'
+	c := NewAnyCacher[any]()
+
+	myStruct := &MyStruct{"MySuperStruct"}
+
+	c.Set("MySuperStruct", myStruct)
+
+	myRawCachedStruct, found := c.Get("MySuperStruct")
+
+	if found {
+		// Casting the retrieved object back to its original type
+		myCachedStruct := myRawCachedStruct.(*MyStruct)
+		fmt.Printf("%s", myCachedStruct.Name)
+	} else {
+		fmt.Printf("Error: MySuperStruct not found in cache")
+	}
+
+	// Output:
+	// MySuperStruct
+}
diff --git a/test/typeparam/issue50109.out b/test/typeparam/issue50109.out
new file mode 100644
index 0000000000..7d6ecc0c6d
--- /dev/null
+++ b/test/typeparam/issue50109.out
@@ -0,0 +1 @@
+MySuperStruct
\ No newline at end of file