cmd/gc: fix import of package with var func returning _

Fixes #8280.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews, r
https://golang.org/cl/146240043

src/cmd/gc/fmt.c

@@ -1108,16 +1108,11 @@ exprfmt(Fmt *f, Node *n, int prec)
 
 	case ONAME:
 		// Special case: name used as local variable in export.
-		switch(n->class&~PHEAP){
-		case PAUTO:
-		case PPARAM:
-		case PPARAMOUT:
-			// _ becomes ~b%d internally; print as _ for export
-			if(fmtmode == FExp && n->sym && n->sym->name[0] == '~' && n->sym->name[1] == 'b')
-				return fmtprint(f, "_");
-			if(fmtmode == FExp && n->sym && !isblank(n) && n->vargen > 0)
-				return fmtprint(f, "%S·%d", n->sym, n->vargen);
-		}
+		// _ becomes ~b%d internally; print as _ for export
+		if(fmtmode == FExp && n->sym && n->sym->name[0] == '~' && n->sym->name[1] == 'b')
+			return fmtprint(f, "_");
+		if(fmtmode == FExp && n->sym && !isblank(n) && n->vargen > 0)
+			return fmtprint(f, "%S·%d", n->sym, n->vargen);
 
 		// Special case: explicit name of func (*T) method(...) is turned into pkg.(*T).method,
 		// but for export, this should be rendered as (*pkg.T).meth.

test/fixedbugs/issue8280.dir/a.go

+package a
+
+var Bar = func() (_ int) { return 0 }

test/fixedbugs/issue8280.dir/b.go

+package b
+
+import "./a"
+
+var foo = a.Bar

test/fixedbugs/issue8280.go

+// compiledir
+
+// Copyright 2014 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 8280: cannot import package exporting a func var returning a result named _
+
+package ignored