cmd/compile: revert internal parameter rename (from ".anonX" to "") before export

In the old binary export format, parameter names for parameter lists
which contained only types where never written, so this problem didn't
come up.

Fixes #25101.

Change-Id: Ia8b817f7f467570b05f88d584e86b6ef4acdccc6
Reviewed-on: https://go-review.googlesource.com/116376Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -778,15 +778,26 @@ func functypefield0(t *types.Type, this *types.Field, in, out []*types.Field) {
 
 // origSym returns the original symbol written by the user.
 func origSym(s *types.Sym) *types.Sym {
-	if s != nil && s.Name[0] == '~' {
+	if s == nil {
+		return nil
+	}
+
+	if len(s.Name) > 1 && s.Name[0] == '~' {
 		switch s.Name[1] {
 		case 'r': // originally an unnamed result
-			s = nil
+			return nil
 		case 'b': // originally the blank identifier _
 			// TODO(mdempsky): Does s.Pkg matter here?
-			s = nblank.Sym
+			return nblank.Sym
 		}
+		return s
 	}
+
+	if strings.HasPrefix(s.Name, ".anon") {
+		// originally an unnamed or _ name (see subr.go: structargs)
+		return nil
+	}
+
 	return s
 }
 

test/fixedbugs/issue25101.go

+// compile
+
+// Copyright 2018 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.
+
+// Indexed export format must not crash when writing
+// the anonymous parameter for m.
+
+package p
+
+var x interface {
+	m(int)
+}
+
+var M = x.m