cmd/compile: remove special-casing of blank in types.sconv{,2}

I'm not sure why blank was special-cased here before, but it's
wrong. Blank is a non-exported identifier, and writing it out without
package-qualification can result in linker symbol collisions.

Fixes #47087.

Change-Id: Ie600037c8e54e3d4fdaeec21e2ca212badbd830b
Reviewed-on: https://go-review.googlesource.com/c/go/+/333163
Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

Author: mdempsky

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

@@ -109,10 +109,6 @@ func sconv(s *Sym, verb rune, mode fmtMode) string {
 		return "<S>"
 	}
 
-	if s.Name == "_" {
-		return "_"
-	}
-
 	q := pkgqual(s.Pkg, verb, mode)
 	if q == "" {
 		return s.Name
@@ -136,10 +132,6 @@ func sconv2(b *bytes.Buffer, s *Sym, verb rune, mode fmtMode) {
 		b.WriteString("<S>")
 		return
 	}
-	if s.Name == "_" {
-		b.WriteString("_")
-		return
-	}
 
 	symfmt(b, s, verb, mode)
 }

test/fixedbugs/issue47087.dir/a.go

@@ -0,0 +1,9 @@
+// Copyright 2021 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() interface{} { return struct{ _ []int }{} }
+
+var X = F()

test/fixedbugs/issue47087.dir/b.go

@@ -0,0 +1,9 @@
+// Copyright 2021 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
+
+func F() interface{} { return struct{ _ []int }{} }
+
+var X = F()

test/fixedbugs/issue47087.dir/main.go

@@ -0,0 +1,19 @@
+// Copyright 2021 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
+
+import (
+	"a"
+	"b"
+)
+
+func main() {
+	if a.F() == b.F() {
+		panic("FAIL")
+	}
+	if a.X == b.X {
+		panic("FAIL")
+	}
+}

test/fixedbugs/issue47087.go

@@ -0,0 +1,7 @@
+// rundir
+
+// Copyright 2021 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