cmd/compile: fix expression switches using type parameters

Both the thing we're switching on, as well as the cases we're switching for.
Convert anything containing a type parameter to interface{} before the
comparison happens.

Fixes #51522

Change-Id: I97ba9429ed332cb7d4240cb60f46d42226dcfa5f
Reviewed-on: https://go-review.googlesource.com/c/go/+/391594
Trust: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/ir/stmt.go

@@ -362,7 +362,7 @@ func NewSendStmt(pos src.XPos, ch, value Node) *SendStmt {
 	return n
 }
 
-// A SwitchStmt is a switch statement: switch Init; Expr { Cases }.
+// A SwitchStmt is a switch statement: switch Init; Tag { Cases }.
 type SwitchStmt struct {
 	miniStmt
 	Tag      Node

src/cmd/compile/internal/noder/stencil.go

@@ -1179,6 +1179,26 @@ func (subst *subster) node(n ir.Node) ir.Node {
 			subst.g.newInsts = append(subst.g.newInsts, m.(*ir.ClosureExpr).Func)
 			m.(*ir.ClosureExpr).SetInit(subst.list(x.Init()))
 
+		case ir.OSWITCH:
+			m := m.(*ir.SwitchStmt)
+			if m.Tag != nil && m.Tag.Op() == ir.OTYPESW {
+				break // Nothing to do here for type switches.
+			}
+			if m.Tag != nil && !m.Tag.Type().IsInterface() && m.Tag.Type().HasShape() {
+				// To implement a switch on a value that is or has a type parameter, we first convert
+				// that thing we're switching on to an interface{}.
+				m.Tag = assignconvfn(m.Tag, types.Types[types.TINTER])
+			}
+			for _, c := range m.Cases {
+				for i, x := range c.List {
+					// If we have a case that is or has a type parameter, convert that case
+					// to an interface{}.
+					if !x.Type().IsInterface() && x.Type().HasShape() {
+						c.List[i] = assignconvfn(x, types.Types[types.TINTER])
+					}
+				}
+			}
+
 		}
 		return m
 	}

test/typeparam/issue51522b.go

@@ -0,0 +1,62 @@
+// run
+
+// 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 main
+
+func f[T comparable](i any) {
+	var t T
+
+	switch i {
+	case t:
+		// ok
+	default:
+		println("FAIL: switch i")
+	}
+
+	switch t {
+	case i:
+		// ok
+	default:
+		println("FAIL: switch t")
+	}
+}
+
+type myint int
+
+func (m myint) foo() {
+}
+
+type fooer interface {
+	foo()
+}
+
+type comparableFoo interface {
+	comparable
+	foo()
+}
+
+func g[T comparableFoo](i fooer) {
+	var t T
+
+	switch i {
+	case t:
+		// ok
+	default:
+		println("FAIL: switch i")
+	}
+
+	switch t {
+	case i:
+		// ok
+	default:
+		println("FAIL: switch t")
+	}
+}
+
+func main() {
+	f[int](0)
+	g[myint](myint(0))
+}