[dev.typeparams] go/types: implement ch <- x where ch is of type parameter type

This is a port of CL 333712 to go/types, adjusted for the different
error reporting API and to position errors on the arrows.

Fixes #43671

Change-Id: I7d2de249e86d272c89a046f60e632e75848ff865
Reviewed-on: https://go-review.googlesource.com/c/go/+/335076
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/types/stmt.go

@@ -361,25 +361,33 @@ func (check *Checker) stmt(ctxt stmtContext, s ast.Stmt) {
 		check.errorf(&x, code, "%s %s", &x, msg)
 
 	case *ast.SendStmt:
-		var ch, x operand
+		var ch, val operand
 		check.expr(&ch, s.Chan)
-		check.expr(&x, s.Value)
+		check.expr(&val, s.Value)
-		if ch.mode == invalid || x.mode == invalid {
+		if ch.mode == invalid || val.mode == invalid {
 			return
 		}
-
+		var elem Type
-		tch := asChan(ch.typ)
+		if !underIs(ch.typ, func(u Type) bool {
-		if tch == nil {
+			uch, _ := u.(*Chan)
-			check.invalidOp(inNode(s, s.Arrow), _InvalidSend, "cannot send to non-chan type %s", ch.typ)
+			if uch == nil {
+				check.invalidOp(inNode(s, s.Arrow), _InvalidSend, "cannot send to non-channel %s", &ch)
+				return false
+			}
+			if uch.dir == RecvOnly {
+				check.invalidOp(inNode(s, s.Arrow), _InvalidSend, "cannot send to receive-only channel %s", &ch)
+				return false
+			}
+			if elem != nil && !Identical(uch.elem, elem) {
+				check.invalidOp(inNode(s, s.Arrow), _Todo, "channels of %s must have the same element type", &ch)
+				return false
+			}
+			elem = uch.elem
+			return true
+		}) {
 			return
 		}
-
+		check.assignment(&val, elem, "send")
-		if tch.dir == RecvOnly {
-			check.invalidOp(inNode(s, s.Arrow), _InvalidSend, "cannot send to receive-only type %s", tch)
-			return
-		}
-
-		check.assignment(&x, tch.elem, "send")
 
 	case *ast.IncDecStmt:
 		var op token.Token

src/go/types/testdata/fixedbugs/issue47115.go2

@@ -0,0 +1,40 @@
+// 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 p
+
+type C0 interface{ int }
+type C1 interface{ chan int }
+type C2 interface{ chan int | <-chan int }
+type C3 interface{ chan int | chan float32 }
+type C4 interface{ chan int | chan<- int }
+type C5[T any] interface{ ~chan T | chan<- T }
+
+func _[T any](ch T) {
+	ch <- /* ERROR cannot send to non-channel */ 0
+}
+
+func _[T C0](ch T) {
+	ch <- /* ERROR cannot send to non-channel */ 0
+}
+
+func _[T C1](ch T) {
+	ch <- 0
+}
+
+func _[T C2](ch T) {
+	ch  <-/* ERROR cannot send to receive-only channel */ 0
+}
+
+func _[T C3](ch T) {
+	ch <- /* ERROR channels of ch .* must have the same element type */ 0
+}
+
+func _[T C4](ch T) {
+	ch <- 0
+}
+
+func _[T C5[X], X any](ch T, x X) {
+	ch <- x
+}