go/doc: treat _ consts as exported

Fixes #5397.

LGTM=adg
R=gri, adg
CC=golang-codereviews, rsc
https://golang.org/cl/144110044

src/go/doc/exports.go

@@ -6,15 +6,18 @@
 
 package doc
 
-import "go/ast"
+import (
+	"go/ast"
+	"go/token"
+)
 
 // filterIdentList removes unexported names from list in place
 // and returns the resulting list.
 //
-func filterIdentList(list []*ast.Ident) []*ast.Ident {
+func filterIdentList(list []*ast.Ident, blankOk bool) []*ast.Ident {
 	j := 0
 	for _, x := range list {
-		if ast.IsExported(x.Name) {
+		if ast.IsExported(x.Name) || (blankOk && x.Name == "_") {
 			list[j] = x
 			j++
 		}
@@ -74,7 +77,7 @@ func (r *reader) filterFieldList(parent *namedType, fields *ast.FieldList, ityp
 				r.remember(ityp)
 			}
 		} else {
-			field.Names = filterIdentList(field.Names)
+			field.Names = filterIdentList(field.Names, false)
 			if len(field.Names) < n {
 				removedFields = true
 			}
@@ -136,13 +139,13 @@ func (r *reader) filterType(parent *namedType, typ ast.Expr) {
 	}
 }
 
-func (r *reader) filterSpec(spec ast.Spec) bool {
+func (r *reader) filterSpec(spec ast.Spec, tok token.Token) bool {
 	switch s := spec.(type) {
 	case *ast.ImportSpec:
 		// always keep imports so we can collect them
 		return true
 	case *ast.ValueSpec:
-		s.Names = filterIdentList(s.Names)
+		s.Names = filterIdentList(s.Names, tok == token.CONST)
 		if len(s.Names) > 0 {
 			r.filterType(nil, s.Type)
 			return true
@@ -159,10 +162,10 @@ func (r *reader) filterSpec(spec ast.Spec) bool {
 	return false
 }
 
-func (r *reader) filterSpecList(list []ast.Spec) []ast.Spec {
+func (r *reader) filterSpecList(list []ast.Spec, tok token.Token) []ast.Spec {
 	j := 0
 	for _, s := range list {
-		if r.filterSpec(s) {
+		if r.filterSpec(s, tok) {
 			list[j] = s
 			j++
 		}
@@ -173,7 +176,7 @@ func (r *reader) filterSpecList(list []ast.Spec) []ast.Spec {
 func (r *reader) filterDecl(decl ast.Decl) bool {
 	switch d := decl.(type) {
 	case *ast.GenDecl:
-		d.Specs = r.filterSpecList(d.Specs)
+		d.Specs = r.filterSpecList(d.Specs, d.Tok)
 		return len(d.Specs) > 0
 	case *ast.FuncDecl:
 		// ok to filter these methods early because any

src/go/doc/testdata/blank.0.golden

+// Package blank is a go/doc test for the handling of _. See issue ...
+PACKAGE blank
+
+IMPORTPATH
+	testdata/blank
+
+FILENAMES
+	testdata/blank.go
+
+CONSTANTS
+	// Package constants. 
+	const (
+		_	int	= iota
+		I1
+		I2
+	)
+
+
+TYPES
+	// S has a padding field. 
+	type S struct {
+		H	uint32
+	
+		A	uint8
+		// contains filtered or unexported fields
+	}
+
+	// 
+	type T int
+
+	// T constants. 
+	const (
+		_	T	= iota
+		T1
+		T2
+	)
+

src/go/doc/testdata/blank.1.golden

+// Package blank is a go/doc test for the handling of _. See issue ...
+PACKAGE blank
+
+IMPORTPATH
+	testdata/blank
+
+FILENAMES
+	testdata/blank.go
+
+CONSTANTS
+	// Package constants. 
+	const (
+		_	int	= iota
+		I1
+		I2
+	)
+
+
+VARIABLES
+	// 
+	var _ = T(55)
+
+
+FUNCTIONS
+	// 
+	func _()
+
+
+TYPES
+	// S has a padding field. 
+	type S struct {
+		H	uint32
+		_	uint8
+		A	uint8
+	}
+
+	// 
+	type T int
+
+	// T constants. 
+	const (
+		_	T	= iota
+		T1
+		T2
+	)
+

src/go/doc/testdata/blank.2.golden

+// Package blank is a go/doc test for the handling of _. See issue ...
+PACKAGE blank
+
+IMPORTPATH
+	testdata/blank
+
+FILENAMES
+	testdata/blank.go
+
+CONSTANTS
+	// Package constants. 
+	const (
+		_	int	= iota
+		I1
+		I2
+	)
+
+
+TYPES
+	// S has a padding field. 
+	type S struct {
+		H	uint32
+	
+		A	uint8
+		// contains filtered or unexported fields
+	}
+
+	// 
+	type T int
+
+	// T constants. 
+	const (
+		_	T	= iota
+		T1
+		T2
+	)
+

src/go/doc/testdata/blank.go

+// 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.
+
+// Package blank is a go/doc test for the handling of _.
+// See issue 5397.
+package blank
+
+type T int
+
+// T constants.
+const (
+	_ T = iota
+	T1
+	T2
+)
+
+// Package constants.
+const (
+	_ int = iota
+	I1
+	I2
+)
+
+// Blanks not in doc output:
+
+// S has a padding field.
+type S struct {
+	H uint32
+	_ uint8
+	A uint8
+}
+
+func _() {}
+
+type _ T
+
+var _ = T(55)