internal/goexperiment: add aliastypeparams GOEXPERIMENT flag

For #46477.

Change-Id: Ic917272cb2cd28dcb39f173b3cdcfb72e52eae2d
Reviewed-on: https://go-review.googlesource.com/c/go/+/586955
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Austin Clements <austin@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Robert Griesemer <gri@google.com>

src/internal/goexperiment/exp_aliastypeparams_off.go

@@ -0,0 +1,8 @@
+// Code generated by mkconsts.go. DO NOT EDIT.
+
+//go:build !goexperiment.aliastypeparams
+
+package goexperiment
+
+const AliasTypeParams = false
+const AliasTypeParamsInt = 0

src/internal/goexperiment/exp_aliastypeparams_on.go

@@ -0,0 +1,8 @@
+// Code generated by mkconsts.go. DO NOT EDIT.
+
+//go:build goexperiment.aliastypeparams
+
+package goexperiment
+
+const AliasTypeParams = true
+const AliasTypeParamsInt = 1

src/internal/goexperiment/flags.go

@@ -110,4 +110,9 @@ type Flags struct {
 
 	// RangeFunc enables range over func.
 	RangeFunc bool
+
+	// AliasTypeParams enables type parameters for alias types.
+	// Requires that gotypesalias=1 is set with GODEBUG.
+	// This flag will be removed with Go 1.24.
+	AliasTypeParams bool
 }