[dev.typealias] cmd/compile, go/types, go/importer: various alias related fixes

cmd/compile:
- remove crud from prior alias implementation
- better comments in places

go/types:
- fix TypeName.IsAlias predicate
- more tests

go/importer (go/internal/gcimporter15):
- handle "@" format for anonymous fields using aliases
  (currently tested indirectly via x/tools/gcimporter15 tests)

For #18130.

Change-Id: I23a6d4e3a4c2a5c1ae589513da73fde7cad5f386
Reviewed-on: https://go-review.googlesource.com/35101Reviewed-by: Alan Donovan <adonovan@google.com>

src/cmd/compile/internal/gc/bexport.go

@@ -447,30 +447,6 @@ func unidealType(typ *Type, val Val) *Type {
 }
 
 func (p *exporter) obj(sym *Sym) {
-	if sym.Flags&SymAlias != 0 {
-		p.tag(aliasTag)
-		p.pos(nil) // TODO(gri) fix position information
-		// Aliases can only be exported from the package that
-		// declares them (aliases to aliases are resolved to the
-		// original object, and so are uses of aliases in inlined
-		// exported function bodies). Thus, we only need the alias
-		// name without package qualification.
-		if sym.Pkg != localpkg {
-			Fatalf("exporter: export of non-local alias: %v", sym)
-		}
-		p.string(sym.Name)
-		orig := sym.Def.Sym
-		if orig.Flags&SymAlias != 0 {
-			Fatalf("exporter: original object %v marked as alias", sym)
-		}
-		p.qualifiedName(orig)
-		return
-	}
-
-	if sym != sym.Def.Sym {
-		Fatalf("exporter: exported object %v is not original %v", sym, sym.Def.Sym)
-	}
-
 	// Exported objects may be from different packages because they
 	// may be re-exported via an exported alias or as dependencies in
 	// exported inlined function bodies. Thus, exported object names
@@ -885,15 +861,15 @@ func (p *exporter) fieldName(t *Field) {
 	name := t.Sym.Name
 	if t.Embedded != 0 {
 		// anonymous field - we distinguish between 3 cases:
-		// 1) field name matches base type name and name is exported
-		// 2) field name matches base type name and name is not exported
-		// 3) field name doesn't match base type name (type name is alias)
+		// 1) field name matches base type name and is exported
+		// 2) field name matches base type name and is not exported
+		// 3) field name doesn't match base type name (alias name)
 		bname := basetypeName(t.Type)
 		if name == bname {
 			if exportname(name) {
-				name = "" // 1) we don't need to know the name
+				name = "" // 1) we don't need to know the field name or package
 			} else {
-				name = "?" // 2) use unexported name to force package export
+				name = "?" // 2) use unexported name "?" to force package export
 			}
 		} else {
 			// 3) indicate alias and export name as is
@@ -920,11 +896,10 @@ func basetypeName(t *Type) string {
 	if s == nil && t.IsPtr() {
 		s = t.Elem().Sym // deref
 	}
-	// s should exist, but be conservative
 	if s != nil {
 		return s.Name
 	}
-	return ""
+	return "" // unnamed type
 }
 
 func (p *exporter) paramList(params *Type, numbered bool) {

src/cmd/compile/internal/gc/bimport.go

@@ -582,7 +582,7 @@ func (p *importer) field() *Field {
 
 	f := newField()
 	if sym.Name == "" {
-		// anonymous field - typ must be T or *T and T must be a type name
+		// anonymous field: typ must be T or *T and T must be a type name
 		s := typ.Sym
 		if s == nil && typ.IsPtr() {
 			s = typ.Elem().Sym // deref
@@ -590,6 +590,7 @@ func (p *importer) field() *Field {
 		sym = sym.Pkg.Lookup(s.Name)
 		f.Embedded = 1
 	} else if sym.Flags&SymAlias != 0 {
+		// anonymous field: we have an explicit name because it's an alias
 		f.Embedded = 1
 	}
 
@@ -635,13 +636,13 @@ func (p *importer) fieldName() *Sym {
 	var flag SymFlags
 	switch name {
 	case "":
-		// field name is exported - nothing to do
+		// 1) field name matches base type name and is exported: nothing to do
 	case "?":
-		// field name is not exported - need package
+		// 2) field name matches base type name and is not exported: need package
 		name = ""
 		pkg = p.pkg()
 	case "@":
-		// field name doesn't match type name (alias)
+		// 3) field name doesn't match base type name (alias name): need name and possibly package
 		name = p.string()
 		flag = SymAlias
 		fallthrough

src/go/internal/gcimporter/bimport.go

@@ -341,9 +341,7 @@ var (
 
 func (p *importer) qualifiedName() (pkg *types.Package, name string) {
 	name = p.string()
-	if name != "" {
-		pkg = p.pkg()
-	}
+	pkg = p.pkg()
 	return
 }
 
@@ -556,7 +554,7 @@ func (p *importer) fieldList(parent *types.Package) (fields []*types.Var, tags [
 
 func (p *importer) field(parent *types.Package) (*types.Var, string) {
 	pos := p.pos()
-	pkg, name := p.fieldName(parent)
+	pkg, name, alias := p.fieldName(parent)
 	typ := p.typ(parent)
 	tag := p.string()
 
@@ -570,9 +568,12 @@ func (p *importer) field(parent *types.Package) (*types.Var, string) {
 		case *types.Named:
 			name = typ.Obj().Name()
 		default:
-			errorf("anonymous field expected")
+			errorf("named base type expected")
 		}
 		anonymous = true
+	} else if alias {
+		// anonymous field: we have an explicit name because it's an alias
+		anonymous = true
 	}
 
 	return types.NewField(pos, pkg, name, typ, anonymous), tag
@@ -590,41 +591,42 @@ func (p *importer) methodList(parent *types.Package) (methods []*types.Func) {
 
 func (p *importer) method(parent *types.Package) *types.Func {
 	pos := p.pos()
-	pkg, name := p.fieldName(parent)
+	pkg, name, _ := p.fieldName(parent)
 	params, isddd := p.paramList()
 	result, _ := p.paramList()
 	sig := types.NewSignature(nil, params, result, isddd)
 	return types.NewFunc(pos, pkg, name, sig)
 }
 
-func (p *importer) fieldName(parent *types.Package) (*types.Package, string) {
-	name := p.string()
-	pkg := parent
+func (p *importer) fieldName(parent *types.Package) (pkg *types.Package, name string, alias bool) {
+	name = p.string()
+	pkg = parent
 	if pkg == nil {
 		// use the imported package instead
 		pkg = p.pkgList[0]
 	}
 	if p.version == 0 && name == "_" {
 		// version 0 didn't export a package for _ fields
-		return pkg, name
+		return
 	}
 	switch name {
 	case "":
-		// field name is exported - nothing to do
+		// 1) field name matches base type name and is exported: nothing to do
 	case "?":
-		// field name is not exported - need package
+		// 2) field name matches base type name and is not exported: need package
 		name = ""
 		pkg = p.pkg()
 	case "@":
-		// field name doesn't match type name (alias)
+		// 3) field name doesn't match type name (alias)
 		name = p.string()
+		alias = true
 		fallthrough
 	default:
 		if !exported(name) {
 			pkg = p.pkg()
 		}
 	}
-	return pkg, name
+	return
 }
 
 func (p *importer) paramList() (*types.Tuple, bool) {

src/go/types/object.go

@@ -163,23 +163,19 @@ func NewTypeName(pos token.Pos, pkg *Package, name string, typ Type) *TypeName {
 	return &TypeName{object{nil, pos, pkg, name, typ, 0, token.NoPos}}
 }
 
+// IsAlias reports whether obj is an alias name for a type.
 func (obj *TypeName) IsAlias() bool {
 	switch t := obj.typ.(type) {
 	case nil:
 		return false
 	case *Basic:
-		// It would seem that we should be able to look for different names here;
-		// but the names of universeByte/Rune are "byte" and "rune", respectively.
-		// We do this so that we get better error messages. However, general alias
-		// types don't have that name information and thus behave differently when
-		// reporting errors (we won't see the alias name, only the original name).
-		// Maybe we should remove the special handling for the predeclared types
-		// as well to be consistent (at the cost of slightly less clear error
-		// messages when byte/rune are involved).
-		// This also plays out in the implementation of the Identical(Type, Type)
-		// predicate.
-		// TODO(gri) consider possible clean up
-		return t == universeByte || t == universeRune
+		// Any user-defined type name for a basic type is an alias for a
+		// basic type (because basic types are pre-declared in the Universe
+		// scope, outside any package scope), and so is any type name with
+		// a different name than the name of the basic type it refers to.
+		// Additionaly, we need to look for "byte" and "rune" because they
+		// are aliases but have the same names (for better error messages).
+		return obj.pkg != nil || t.name != obj.name || t == universeByte || t == universeRune
 	case *Named:
 		return obj != t.obj
 	default:

src/go/types/object_test.go

@@ -21,16 +21,23 @@ func TestIsAlias(t *testing.T) {
 	}
 
 	// various other types
-	t0 := NewTypeName(0, nil, "t0", nil)
-	check(t0, false) // no type yet
-
-	t1 := NewTypeName(0, nil, "t1", nil)
+	pkg := NewPackage("p", "p")
+	t1 := NewTypeName(0, pkg, "t1", nil)
 	n1 := NewNamed(t1, new(Struct), nil)
-	check(t1, false) // type name refers to named type and vice versa
-
-	t2 := NewTypeName(0, nil, "t2", new(Interface))
-	check(t2, true) // type name refers to unnamed type
-
-	t3 := NewTypeName(0, nil, "t3", n1)
-	check(t3, true) // type name refers to named type with different type name (true alias)
+	for _, test := range []struct {
+		name  *TypeName
+		alias bool
+	}{
+		{NewTypeName(0, nil, "t0", nil), false}, // no type yet
+		{NewTypeName(0, pkg, "t0", nil), false}, // no type yet
+		{t1, false},                             // type name refers to named type and vice versa
+		{NewTypeName(0, nil, "t2", new(Interface)), true}, // type name refers to unnamed type
+		{NewTypeName(0, pkg, "t3", n1), true},             // type name refers to named type with different type name
+		{NewTypeName(0, nil, "t4", Typ[Int32]), true},     // type name refers to basic type with different name
+		{NewTypeName(0, nil, "int32", Typ[Int32]), false}, // type name refers to basic type with same name
+		{NewTypeName(0, pkg, "int32", Typ[Int32]), true},  // type name is declared in user-defined package (outside Universe)
+		{NewTypeName(0, nil, "rune", Typ[Rune]), true},    // type name refers to basic type rune which is an alias already
+	} {
+		check(test.name, test.alias)
+	}
 }