Map: Work around https://github.com/golang/go/issues/56548

3 files changed, 35 insertions, 1 deletions

diff --git a/map.go b/map.go
index 5b53748..6bb1170 100644
--- a/map.go
+++ b/map.go
@@ -8,7 +8,23 @@ import (
 	"sync"
 )
 
-type Map[K comparable, V any] struct {
+// Map is a type-safe equivalent of the standard library's sync.Map.
+//
+// With versions of Go prior to Go 1.20, Map is specified too loosely,
+// as
+//
+//	Map[K any, V any]
+//
+// while with Go 1.20 and later, Map is specified as
+//
+//	Map[K comparable, V any]
+//
+// This is because with Go versions prior to 1.20, 'comparable' was
+// overly strict, disallowing many types that are valid map-keys (see
+// https://github.com/golang/go/issues/56548).  The type used as K in
+// a Map older versions of Go must be a valid map-key type, even
+// though the type specification of Map does not enforce that.
+type Map[K mapkey, V any] struct {
 	inner sync.Map
 }
 
diff --git a/map_go118.go b/map_go118.go
new file mode 100644
index 0000000..5446c88
--- /dev/null
+++ b/map_go118.go
@@ -0,0 +1,9 @@
+// Copyright (C) 2023  Luke Shumaker <lukeshu@lukeshu.com>
+//
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+//go:build !go1.20
+
+package typedsync
+
+type mapkey = any
diff --git a/map_go120.go b/map_go120.go
new file mode 100644
index 0000000..0d4ff5b
--- /dev/null
+++ b/map_go120.go
@@ -0,0 +1,9 @@
+// Copyright (C) 2023  Luke Shumaker <lukeshu@lukeshu.com>
+//
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+//go:build go1.20
+
+package typedsync
+
+type mapkey = comparable