cmd/compile: fix corner case in DWARF inline info generation

The helper routine for returning pre-inlining parameter declarations
wasn't properly handling the case where you have more than one
parameter named "_" in a function signature; this triggered a map
collision later on when the function was inlined and DWARF was
generated for the inlined routine instance.

Fixes #23179.

Change-Id: I12e5d6556ec5ce08e982a6b53666a4dcc1d22201
Reviewed-on: https://go-review.googlesource.com/84755
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

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

@@ -599,7 +599,9 @@ func preInliningDcls(fnsym *obj.LSym) []*Node {
 	}
 	for _, n := range dcl {
 		c := n.Sym.Name[0]
-		if c == '.' || n.Type.IsUntyped() {
+		// Avoid reporting "_" parameters, since if there are more tham
+		// one, it can result in a collision later on, as in #23179.
+		if unversion(n.Sym.Name) == "_" || c == '.' || n.Type.IsUntyped() {
 			continue
 		}
 		rdcl = append(rdcl, n)

test/fixedbugs/issue23179.dir/a.go

@@ -0,0 +1,9 @@
+// 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.
+
+package a
+
+func F(x int, _ int, _ bool) int {
+	return x
+}

test/fixedbugs/issue23179.dir/b.go

@@ -0,0 +1,11 @@
+// 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.
+
+package b
+
+import "a"
+
+func G(x int) int {
+	return a.F(x, 1, false)
+}

test/fixedbugs/issue23179.go

@@ -0,0 +1,7 @@
+// compiledir
+
+// 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.
+
+package ignored