cmd/compile: avoid checkwidth of [...] arrays

Fixes #20333

Change-Id: I0653cc859076f146d8ea8f5bd55cb22b0b8d987f
Reviewed-on: https://go-review.googlesource.com/43290
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/gc/typecheck.go

@@ -498,7 +498,12 @@ OpSwitch:
 			ok |= Etype
 			n.Op = OTYPE
 			n.Type = types.NewPtr(l.Type)
-			checkwidth(l.Type) // ensure this gets dowidth'd for the backend
+			// Ensure l.Type gets dowidth'd for the backend. Issue 20174.
+			// Don't checkwidth [...] arrays, though, since they
+			// will be replaced by concrete-sized arrays. Issue 20333.
+			if !l.Type.IsDDDArray() {
+				checkwidth(l.Type)
+			}
 			n.Left = nil
 			break OpSwitch
 		}

test/fixedbugs/issue20333.go

+// compile
+
+// Copyright 2017 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.
+
+// Issue 20333: early checkwidth of [...] arrays led to compilation errors.
+
+package main
+
+import "fmt"
+
+func main() {
+	fmt.Println(&[...]string{"abc", "def", "ghi"})
+}