go/parser: parse incomplete selection "fmt." as a blank selection "fmt._"

Formerly it would return a BadExpr.

This prevents partial syntax from being discarded, and makes the error
recovery logic more consistent with other places where an identifier
was expected but not found.

+ test

Change-Id: I223c0c0589e7ceb7207ae951b8f71b9275a1eb73
Reviewed-on: https://go-review.googlesource.com/10269
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/parser/interface.go

@@ -91,7 +91,10 @@ func ParseFile(fset *token.FileSet, filename string, src interface{}, mode Mode)
 	var p parser
 	defer func() {
 		if e := recover(); e != nil {
-			_ = e.(bailout) // re-panics if it's not a bailout
+			// resume same panic if it's not a bailout
+			if _, ok := e.(bailout); !ok {
+				panic(e)
+			}
 		}
 
 		// set result values

src/go/parser/parser.go

@@ -1472,7 +1472,8 @@ L:
 				pos := p.pos
 				p.errorExpected(pos, "selector or type assertion")
 				p.next() // make progress
-				x = &ast.BadExpr{From: pos, To: p.pos}
+				sel := &ast.Ident{NamePos: pos, Name: "_"}
+				x = &ast.SelectorExpr{X: x, Sel: sel}
 			}
 		case token.LBRACK:
 			if lhs {

src/go/parser/parser_test.go

@@ -492,3 +492,42 @@ func TestIssue9979(t *testing.T) {
 		})
 	}
 }
+
+// TestIncompleteSelection ensures that an incomplete selector
+// expression is parsed as a (blank) *ast.SelectorExpr, not a
+// *ast.BadExpr.
+func TestIncompleteSelection(t *testing.T) {
+	for _, src := range []string{
+		"package p; var _ = fmt.",             // at EOF
+		"package p; var _ = fmt.\ntype X int", // not at EOF
+	} {
+		fset := token.NewFileSet()
+		f, err := ParseFile(fset, "", src, 0)
+		if err == nil {
+			t.Errorf("ParseFile(%s) succeeded unexpectedly", src)
+			continue
+		}
+
+		const wantErr = "expected selector or type assertion"
+		if !strings.Contains(err.Error(), wantErr) {
+			t.Errorf("ParseFile returned wrong error %q, want %q", err, wantErr)
+		}
+
+		var sel *ast.SelectorExpr
+		ast.Inspect(f, func(n ast.Node) bool {
+			if n, ok := n.(*ast.SelectorExpr); ok {
+				sel = n
+			}
+			return true
+		})
+		if sel == nil {
+			t.Error("found no *ast.SelectorExpr")
+			continue
+		}
+		const wantSel = "&{fmt _}"
+		if fmt.Sprint(sel) != wantSel {
+			t.Errorf("found selector %s, want %s", sel, wantSel)
+			continue
+		}
+	}
+}