[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>

1 files changed, 27 insertions, 9 deletions

diff --git a/src/go/types/expr.go b/src/go/types/expr.go
index 46f6e33463..751a360890 100644
--- a/src/go/types/expr.go
+++ b/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
 	}