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

This is a port of CL 333709 to go/types, adjusted for the different
error API.

Fixes #43671

Change-Id: Ifd340149bfbcabe401cec74398cb83f2ae812e51
Reviewed-on: https://go-review.googlesource.com/c/go/+/335075
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/expr.go

@@ -144,6 +144,14 @@ var op2str2 = [...]string{
 	token.SHL: "shift",
 }
 
+func underIs(typ Type, f func(Type) bool) bool {
+	u := under(typ)
+	if tpar, _ := u.(*TypeParam); tpar != nil {
+		return tpar.underIs(f)
+	}
+	return f(u)
+}
+
 // The unary expression e may be nil. It's passed in for better error messages only.
 func (check *Checker) unary(x *operand, e *ast.UnaryExpr) {
 	check.expr(x, e.X)
@@ -164,19 +172,29 @@ func (check *Checker) unary(x *operand, e *ast.UnaryExpr) {
 		return
 
 	case token.ARROW:
-		typ := asChan(x.typ)
-		if typ == nil {
-			check.invalidOp(x, _InvalidReceive, "cannot receive from non-channel %s", x)
-			x.mode = invalid
-			return
-		}
-		if typ.dir == SendOnly {
-			check.invalidOp(x, _InvalidReceive, "cannot receive from send-only channel %s", x)
+		var elem Type
+		if !underIs(x.typ, func(u Type) bool {
+			ch, _ := u.(*Chan)
+			if ch == nil {
+				check.invalidOp(x, _InvalidReceive, "cannot receive from non-channel %s", x)
+				return false
+			}
+			if ch.dir == SendOnly {
+				check.invalidOp(x, _InvalidReceive, "cannot receive from send-only channel %s", x)
+				return false
+			}
+			if elem != nil && !Identical(ch.elem, elem) {
+				check.invalidOp(x, _Todo, "channels of %s must have the same element type", x)
+				return false
+			}
+			elem = ch.elem
+			return true
+		}) {
 			x.mode = invalid
 			return
 		}
 		x.mode = commaok
-		x.typ = typ.elem
+		x.typ = elem
 		check.hasCallOrRecv = true
 		return
 	}

src/go/types/testdata/fixedbugs/issue43671.go2

@@ -0,0 +1,58 @@
+// 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 receive from non-channel
+}
+
+func _[T C0](ch T) {
+	<-ch // ERROR cannot receive from non-channel
+}
+
+func _[T C1](ch T) {
+	<-ch
+}
+
+func _[T C2](ch T) {
+	<-ch
+}
+
+func _[T C3](ch T) {
+	<-ch // ERROR channels of ch .* must have the same element type
+}
+
+func _[T C4](ch T) {
+	<-ch // ERROR cannot receive from send-only channel
+}
+
+func _[T C5[X], X any](ch T, x X) {
+	x = <-ch
+}
+
+// test case from issue, slightly modified
+type RecvChan[T any] interface {
+	~chan T | ~<-chan T
+}
+
+func _[T any, C RecvChan[T]](ch C) T {
+	return <-ch
+}
+
+func f[T any, C interface{ chan T }](ch C) T {
+	return <-ch
+}
+
+func _(ch chan int) {
+	var x int = f(ch) // test constraint type inference for this case
+	_ = x
+}