go/doc: propagate types from unexported constants

When constants were declared using unexported constants,
the type information was lost when those constants were filtered out.
This CL propagates the type information of unexported constants
so that it is available for display.

This is a follow-up to CL 144110044, which fixed this problem
specifically for _ constants.

Updates #5397.

Change-Id: I3f0c767a4007d88169a5634ab2870deea4e6a740
Reviewed-on: https://go-review.googlesource.com/2091Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/doc/exports.go

@@ -26,6 +26,17 @@ func filterIdentList(list []*ast.Ident, blankOk bool) []*ast.Ident {
 	return list[0:j]
 }
 
+// hasExportedOrBlankName reports whether list contains any exported or blank names.
+//
+func hasExportedOrBlankName(list []*ast.Ident) bool {
+	for _, x := range list {
+		if x.IsExported() || x.Name == "_" {
+			return true
+		}
+	}
+	return false
+}
+
 // removeErrorField removes anonymous fields named "error" from an interface.
 // This is called when "error" has been determined to be a local name,
 // not the predeclared type.
@@ -165,7 +176,47 @@ func (r *reader) filterSpec(spec ast.Spec, tok token.Token) bool {
 	return false
 }
 
+// copyConstType returns a copy of typ with position pos.
+// typ must be a valid constant type.
+// In practice, only (possibly qualified) identifiers are possible.
+//
+func copyConstType(typ ast.Expr, pos token.Pos) ast.Expr {
+	switch typ := typ.(type) {
+	case *ast.Ident:
+		return &ast.Ident{Name: typ.Name, NamePos: pos}
+	case *ast.SelectorExpr:
+		if id, ok := typ.X.(*ast.Ident); ok {
+			// presumably a qualified identifier
+			return &ast.SelectorExpr{
+				Sel: ast.NewIdent(typ.Sel.Name),
+				X:   &ast.Ident{Name: id.Name, NamePos: pos},
+			}
+		}
+	}
+	return nil // shouldn't happen, but be conservative and don't panic
+}
+
 func (r *reader) filterSpecList(list []ast.Spec, tok token.Token) []ast.Spec {
+	if tok == token.CONST {
+		// Propagate any type information that would get lost otherwise
+		// when unexported constants are filtered.
+		var prevType ast.Expr
+		for _, spec := range list {
+			spec := spec.(*ast.ValueSpec)
+			if spec.Type == nil && prevType != nil {
+				// provide current spec with an explicit type
+				spec.Type = copyConstType(prevType, spec.Pos())
+			}
+			if hasExportedOrBlankName(spec.Names) {
+				// both exported and blank names are preserved
+				// so there's no need to propagate the type
+				prevType = nil
+			} else {
+				prevType = spec.Type
+			}
+		}
+	}
+
 	j := 0
 	for _, s := range list {
 		if r.filterSpec(s, tok) {

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

@@ -4,10 +4,30 @@ PACKAGE blank
 IMPORTPATH
 	testdata/blank
 
+IMPORTS
+	os
+
 FILENAMES
 	testdata/blank.go
 
 CONSTANTS
+	// T constants counting from unexported constants. 
+	const (
+		C1	T
+		C2
+	
+		C3
+	
+		C4	int
+	)
+
+	// Constants with an imported type that needs to be propagated. 
+	const (
+		Default		os.FileMode	= 0644
+		Useless				= 0312
+		WideOpen			= 0777
+	)
+
 	// Package constants. 
 	const (
 		_	int	= iota
@@ -28,7 +48,7 @@ TYPES
 	// 
 	type T int
 
-	// T constants. 
+	// T constants counting from a blank constant. 
 	const (
 		_	T	= iota
 		T1

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

@@ -4,10 +4,25 @@ PACKAGE blank
 IMPORTPATH
 	testdata/blank
 
+IMPORTS
+	os
+
 FILENAMES
 	testdata/blank.go
 
 CONSTANTS
+	// T constants counting from unexported constants. 
+	const (
+		tweedledee	T	= iota
+		tweedledum
+		C1
+		C2
+		alice
+		C3
+		redQueen	int	= iota
+		C4
+	)
+
 	// Package constants. 
 	const (
 		_	int	= iota
@@ -15,6 +30,14 @@ CONSTANTS
 		I2
 	)
 
+	// Constants with an imported type that needs to be propagated. 
+	const (
+		zero		os.FileMode	= 0
+		Default				= 0644
+		Useless				= 0312
+		WideOpen			= 0777
+	)
+
 
 VARIABLES
 	// 
@@ -37,7 +60,7 @@ TYPES
 	// 
 	type T int
 
-	// T constants. 
+	// T constants counting from a blank constant. 
 	const (
 		_	T	= iota
 		T1

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

@@ -4,10 +4,30 @@ PACKAGE blank
 IMPORTPATH
 	testdata/blank
 
+IMPORTS
+	os
+
 FILENAMES
 	testdata/blank.go
 
 CONSTANTS
+	// T constants counting from unexported constants. 
+	const (
+		C1	T
+		C2
+	
+		C3
+	
+		C4	int
+	)
+
+	// Constants with an imported type that needs to be propagated. 
+	const (
+		Default		os.FileMode	= 0644
+		Useless				= 0312
+		WideOpen			= 0777
+	)
+
 	// Package constants. 
 	const (
 		_	int	= iota
@@ -28,7 +48,7 @@ TYPES
 	// 
 	type T int
 
-	// T constants. 
+	// T constants counting from a blank constant. 
 	const (
 		_	T	= iota
 		T1

src/go/doc/testdata/blank.go

@@ -6,15 +6,37 @@
 // See issue 5397.
 package blank
 
+import "os"
+
 type T int
 
-// T constants.
+// T constants counting from a blank constant.
 const (
 	_ T = iota
 	T1
 	T2
 )
 
+// T constants counting from unexported constants.
+const (
+	tweedledee T = iota
+	tweedledum
+	C1
+	C2
+	alice
+	C3
+	redQueen int = iota
+	C4
+)
+
+// Constants with an imported type that needs to be propagated.
+const (
+	zero     os.FileMode = 0
+	Default              = 0644
+	Useless              = 0312
+	WideOpen             = 0777
+)
+
 // Package constants.
 const (
 	_ int = iota