cmd/compile: fix conversion error message for printed slices

Fixes #15055.

Updates exprfmt printing using fmt verb "%v" to check that n.Left
is non-nil before attempting to print it, otherwise we'll print
the nodes in the list using verb "%.v".

Credit to @mdempsky for this approach and for finding
the root cause of the issue.

Change-Id: I20a6464e916dc70d5565e145164bb9553e5d3865
Reviewed-on: https://go-review.googlesource.com/25361Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -1335,14 +1335,15 @@ func (n *Node) exprfmt(s fmt.State, prec int) {
 		OSTRARRAYRUNE,
 		ORUNESTR:
 		if n.Type == nil || n.Type.Sym == nil {
-			fmt.Fprintf(s, "(%v)(%v)", n.Type, n.Left)
-			return
+			fmt.Fprintf(s, "(%v)", n.Type)
+		} else {
+			fmt.Fprintf(s, "%v", n.Type)
 		}
 		if n.Left != nil {
-			fmt.Fprintf(s, "%v(%v)", n.Type, n.Left)
-			return
+			fmt.Fprintf(s, "(%v)", n.Left)
+		} else {
+			fmt.Fprintf(s, "(%.v)", n.List)
 		}
-		fmt.Fprintf(s, "%v(%.v)", n.Type, n.List)
 
 	case OREAL,
 		OIMAG,

test/fixedbugs/issue15055.go

+// errorcheck
+
+// Copyright 2016 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 main
+
+func main() {
+	type name string
+	_ = []byte("abc", "def", 12)    // ERROR "too many arguments to conversion to \[\]byte: \(\[\]byte\)\(.abc., .def., 12\)"
+	_ = string("a", "b", nil)       // ERROR "too many arguments to conversion to string: string\(.a., .b., nil\)"
+	_ = []byte()                    // ERROR "missing argument to conversion to \[\]byte: \(\[\]byte\)\(\)"
+	_ = string()                    // ERROR "missing argument to conversion to string: string\(\)"
+	_ = name("a", 1, 3.3)           // ERROR "too many arguments to conversion to name: name\(.a., 1, 3.3\)"
+	_ = map[string]string(nil, nil) // ERROR "too many arguments to conversion to map\[string\]string: \(map\[string\]string\)\(nil, nil\)"
+}