go/types: report invalid ... in conversions

This is a port of CL 277072 from the dev.typeparams branch.

Fixes #43124

Change-Id: I1424c396dc1ea984ec85b8f31a4d43353bf7e4fc
Reviewed-on: https://go-review.googlesource.com/c/go/+/277352
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/types/call.go

@@ -33,6 +33,10 @@ func (check *Checker) call(x *operand, e *ast.CallExpr) exprKind {
 		case 1:
 			check.expr(x, e.Args[0])
 			if x.mode != invalid {
+				if e.Ellipsis.IsValid() {
+					check.errorf(e.Args[0], _BadDotDotDotSyntax, "invalid use of ... in conversion to %s", T)
+					break
+				}
 				check.conversion(x, T)
 			}
 		default:

src/go/types/fixedbugs/issue43124.src

@@ -0,0 +1,16 @@
+// Copyright 2020 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
+
+var _ = int(0 /* ERROR invalid use of \.\.\. in conversion to int */ ...)
+
+// test case from issue
+
+type M []string
+
+var (
+	x = []string{"a", "b"}
+	_ = M(x /* ERROR invalid use of \.\.\. in conversion to M */ ...)
+)