[dev.typealias] cmd/compile: type-check type alias declarations

Known issues:
- needs many more tests
- duplicate method declarations via type alias names are not detected
- type alias cycle error messages need to be improved
- need to review setup of byte/rune type aliases

For #18130.

Change-Id: Icc2fefad6214e5e56539a9dcb3fe537bf58029f8
Reviewed-on: https://go-review.googlesource.com/35121
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -352,8 +352,8 @@ func export(out *bufio.Writer, trace bool) int {
 			p.tracef("\n")
 		}
 
-		if sym.Flags&SymAlias != 0 {
-			Fatalf("exporter: unexpected alias %v in inlined function body", sym)
+		if sym.isAlias() {
+			Fatalf("exporter: unexpected type alias %v in inlined function body", sym)
 		}
 
 		p.obj(sym)
@@ -486,8 +486,7 @@ func (p *exporter) obj(sym *Sym) {
 			Fatalf("exporter: export of incomplete type %v", sym)
 		}
 
-		const alias = false // TODO(gri) fix this
-		if alias {
+		if sym.isAlias() {
 			p.tag(aliasTag)
 			p.pos(n)
 			p.qualifiedName(sym)

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

@@ -316,10 +316,10 @@ func (p *importer) obj(tag int) {
 		importconst(sym, idealType(typ), nodlit(val))
 
 	case aliasTag:
-		// TODO(gri) hook up type alias
 		p.pos()
-		p.qualifiedName()
-		p.typ()
+		sym := p.qualifiedName()
+		typ := p.typ()
+		importalias(sym, typ)
 
 	case typeTag:
 		p.typ()
@@ -576,7 +576,7 @@ func (p *importer) fieldList() (fields []*Field) {
 
 func (p *importer) field() *Field {
 	p.pos()
-	sym := p.fieldName()
+	sym, alias := p.fieldName()
 	typ := p.typ()
 	note := p.string()
 
@@ -589,8 +589,8 @@ 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
+	} else if alias {
+		// anonymous field: we have an explicit name because it's a type alias
 		f.Embedded = 1
 	}
 
@@ -625,15 +625,15 @@ func (p *importer) method() *Field {
 	return f
 }
 
-func (p *importer) fieldName() *Sym {
+func (p *importer) fieldName() (*Sym, bool) {
 	name := p.string()
 	if p.version == 0 && name == "_" {
 		// version 0 didn't export a package for _ field names
 		// but used the builtin package instead
-		return builtinpkg.Lookup(name)
+		return builtinpkg.Lookup(name), false
 	}
 	pkg := localpkg
-	var flag SymFlags
+	alias := false
 	switch name {
 	case "":
 		// 1) field name matches base type name and is exported: nothing to do
@@ -644,16 +644,14 @@ func (p *importer) fieldName() *Sym {
 	case "@":
 		// 3) field name doesn't match base type name (alias name): need name and possibly package
 		name = p.string()
-		flag = SymAlias
+		alias = true
 		fallthrough
 	default:
 		if !exportname(name) {
 			pkg = p.pkg()
 		}
 	}
-	sym := pkg.Lookup(name)
-	sym.Flags |= flag
-	return sym
+	return pkg.Lookup(name), alias
 }
 
 func (p *importer) methodName() *Sym {

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

@@ -695,10 +695,20 @@ func typedcl0(s *Sym) *Node {
 
 // node n, which was returned by typedcl0
 // is being declared to have uncompiled type t.
-// return the ODCLTYPE node to use.
-func typedcl1(n *Node, t *Node, local bool) *Node {
-	n.Name.Param.Ntype = t
-	n.Local = local
+// returns the ODCLTYPE node to use.
+func typedcl1(n *Node, t *Node, pragma Pragma, alias bool) *Node {
+	if pragma != 0 && alias {
+		yyerror("cannot specify directive with type alias")
+		pragma = 0
+	}
+
+	n.Local = true
+
+	p := n.Name.Param
+	p.Ntype = t
+	p.Pragma = pragma
+	p.Alias = alias
+
 	return nod(ODCLTYPE, n, nil)
 }
 

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

@@ -45,8 +45,8 @@ func exportsym(n *Node) {
 		fmt.Printf("export symbol %v\n", n.Sym)
 	}
 
-	// Ensure original object is on exportlist before aliases.
-	if n.Sym.Flags&SymAlias != 0 {
+	// Ensure original types are on exportlist before type aliases.
+	if n.Sym.isAlias() {
 		exportlist = append(exportlist, n.Sym.Def)
 	}
 
@@ -348,6 +348,27 @@ func importvar(s *Sym, t *Type) {
 	}
 }
 
+// importalias declares symbol s as an imported type alias with type t.
+func importalias(s *Sym, t *Type) {
+	importsym(s, OTYPE)
+	if s.Def != nil && s.Def.Op == OTYPE {
+		if eqtype(t, s.Def.Type) {
+			return
+		}
+		yyerror("inconsistent definition for type alias %v during import\n\t%v (in %q)\n\t%v (in %q)", s, s.Def.Type, s.Importdef.Path, t, importpkg.Path)
+	}
+
+	n := newname(s)
+	n.Op = OTYPE
+	s.Importdef = importpkg
+	n.Type = t
+	declare(n, PEXTERN)
+
+	if Debug['E'] != 0 {
+		fmt.Printf("import type %v = %L\n", s, t)
+	}
+}
+
 func dumpasmhdr() {
 	b, err := bio.Create(asmhdr)
 	if err != nil {

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

@@ -63,9 +63,12 @@ const (
 	SymSiggen
 	SymAsm
 	SymAlgGen
-	SymAlias // alias, original is Sym.Def.Sym
 )
 
+func (sym *Sym) isAlias() bool {
+	return sym.Def != nil && sym.Def.Sym != sym
+}
+
 // The Class of a variable/function describes the "storage class"
 // of a variable or function. During parsing, storage classes are
 // called declaration contexts.
@@ -87,7 +90,7 @@ const (
 // of the compilers arrays.
 //
 // typedef	struct
-// {					// must not move anything
+// {				// must not move anything
 // 	uchar	array[8];	// pointer to data
 // 	uchar	nel[4];		// number of elements
 // 	uchar	cap[4];		// allocated number of elements
@@ -104,7 +107,7 @@ var sizeof_Array int // runtime sizeof(Array)
 // of the compilers strings.
 //
 // typedef	struct
-// {					// must not move anything
+// {				// must not move anything
 // 	uchar	array[8];	// pointer to data
 // 	uchar	nel[4];		// number of elements
 // } String;

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

@@ -927,7 +927,7 @@ func mkpackage(pkgname string) {
 				continue
 			}
 
-			if s.Def.Sym != s && s.Flags&SymAlias == 0 {
+			if s.isAlias() {
 				// throw away top-level name left over
 				// from previous import . "x"
 				if s.Def.Name != nil && s.Def.Name.Pack != nil && !s.Def.Name.Pack.Used && nsyntaxerrors == 0 {

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

@@ -177,21 +177,12 @@ func (p *noder) constDecl(decl *syntax.ConstDecl) []*Node {
 }
 
 func (p *noder) typeDecl(decl *syntax.TypeDecl) *Node {
-	if decl.Alias {
-		yyerror("type alias declarations unimplemented")
-	}
-
 	name := typedcl0(p.name(decl.Name))
-	pragma := Pragma(decl.Pragma)
-	if pragma != 0 && decl.Alias {
-		yyerror("cannot specify directive with type alias")
-		pragma = 0
-	}
-	name.Name.Param.Pragma = pragma
 
+	// decl.Type may be nil but in that case we got a syntax error during parsing
 	typ := p.typeExprOrNil(decl.Type)
 
-	return typedcl1(name, typ, true)
+	return typedcl1(name, typ, Pragma(decl.Pragma), decl.Alias)
 }
 
 func (p *noder) declNames(names []*syntax.Name) []*Node {

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

@@ -27,7 +27,7 @@ type Node struct {
 	// func
 	Func *Func
 
-	// ONAME
+	// ONAME, OTYPE, OPACK, OLABEL, some OLITERAL
 	Name *Name
 
 	Sym *Sym        // various
@@ -59,8 +59,8 @@ type Node struct {
 	Noescape  bool  // func arguments do not escape; TODO(rsc): move Noescape to Func struct (see CL 7360)
 	Walkdef   uint8 // tracks state during typecheckdef; 2 == loop detected
 	Typecheck uint8 // tracks state during typechecking; 2 == loop detected
-	Local     bool
-	IsStatic  bool // whether this Node will be converted to purely static data
+	Local     bool  // type created in this file (see also Type.Local); TODO(gri): move this into flags
+	IsStatic  bool  // whether this Node will be converted to purely static data
 	Initorder uint8
 	Used      bool // for variable/label declared and not used error
 	Isddd     bool // is the argument variadic
@@ -180,14 +180,14 @@ func (n *Node) SetIota(x int64) {
 	n.Xoffset = x
 }
 
-// Name holds Node fields used only by named nodes (ONAME, OPACK, OLABEL, some OLITERAL).
+// Name holds Node fields used only by named nodes (ONAME, OTYPE, OPACK, OLABEL, some OLITERAL).
 type Name struct {
 	Pack      *Node  // real package for import . names
 	Pkg       *Pkg   // pkg for OPACK nodes
 	Heapaddr  *Node  // temp holding heap address of param (could move to Param?)
 	Defn      *Node  // initializing assignment
 	Curfn     *Node  // function for local variables
-	Param     *Param // additional fields for ONAME
+	Param     *Param // additional fields for ONAME, OTYPE
 	Decldepth int32  // declaration loop depth, increased for every loop or label
 	Vargen    int32  // unique name for ONAME within a function.  Function outputs are numbered starting at one.
 	Funcdepth int32
@@ -280,10 +280,11 @@ type Param struct {
 	Innermost *Node
 	Outer     *Node
 
-	// OTYPE pragmas
+	// OTYPE
 	//
 	// TODO: Should Func pragmas also be stored on the Name?
 	Pragma Pragma
+	Alias  bool // node is alias for Ntype
 }
 
 // Func holds Node fields used only with function-like nodes.
@@ -382,7 +383,7 @@ const (
 	ODCLFUNC  // func f() or func (r) f()
 	ODCLFIELD // struct field, interface field, or func/method argument/return value.
 	ODCLCONST // const pi = 3.14
-	ODCLTYPE  // type Int int
+	ODCLTYPE  // type Int int or type Int = int
 
 	ODELETE    // delete(Left, Right)
 	ODOT       // Left.Sym (Left is of struct type)

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

@@ -3578,8 +3578,6 @@ func typecheckdeftype(n *Node) {
 
 	// copy new type and clear fields
 	// that don't come along.
-	// anything zeroed here must be zeroed in
-	// typedcl2 too.
 	copytype(n, t)
 
 ret:
@@ -3758,12 +3756,29 @@ func typecheckdef(n *Node) *Node {
 		n.Name.Defn = typecheck(n.Name.Defn, Etop) // fills in n->type
 
 	case OTYPE:
+		if p := n.Name.Param; p.Alias {
+			// Type alias declaration: Simply use the rhs type - no need
+			// to create a new type.
+			// If we have a syntax error, p.Ntype may be nil.
+			if p.Ntype != nil {
+				p.Ntype = typecheck(p.Ntype, Etype)
+				n.Type = p.Ntype.Type
+				if n.Type == nil {
+					n.Diag = true
+					goto ret
+				}
+				n.Sym.Def = p.Ntype
+			}
+			break
+		}
+
+		// regular type declaration
 		if Curfn != nil {
 			defercheckwidth()
 		}
 		n.Walkdef = 1
 		n.Type = typ(TFORW)
-		n.Type.Sym = n.Sym
+		n.Type.Sym = n.Sym // TODO(gri) this also happens in typecheckdeftype(n) - where should it happen?
 		nerrors0 := nerrors
 		typecheckdeftype(n)
 		if n.Type.Etype == TFORW && nerrors > nerrors0 {
@@ -3771,7 +3786,6 @@ func typecheckdef(n *Node) *Node {
 			// but it was reported. Silence future errors.
 			n.Type.Broke = true
 		}
-
 		if Curfn != nil {
 			resumecheckwidth()
 		}

test/alias2.go

@@ -6,11 +6,6 @@
 
 // Test basic restrictions on type aliases.
 
-// The compiler doesn't implement type aliases yet,
-// so for now we get the same error (unimplemented)
-// everywhere, OR-ed into the ERROR checks.
-// TODO(gri) remove the need for "unimplemented"
-
 package p
 
 import (
@@ -18,41 +13,87 @@ import (
 	. "reflect"
 )
 
+type T0 struct{}
+
 // Valid type alias declarations.
 
-type _ = int           // ERROR "unimplemented"
-type _ = struct{}      // ERROR "unimplemented"
-type _ = reflect.Value // ERROR "unimplemented"
-type _ = Value         // ERROR "unimplemented"
+type _ = T0
+type _ = int
+type _ = struct{}
+type _ = reflect.Value
+type _ = Value
 
 type (
-	a1 = int           // ERROR "unimplemented"
-	a2 = struct{}      // ERROR "unimplemented"
-	a3 = reflect.Value // ERROR "unimplemented"
-	a4 = Value         // ERROR "unimplemented"
+	A0 = T0
+	A1 = int
+	A2 = struct{}
+	A3 = reflect.Value
+	A4 = Value
+	A5 = Value
+
+	N0 A0
 )
 
+// Methods can be declared on the original named type and the alias.
+func (T0) m1() {}
+func (A0) m1() {} // TODO(gri) this should be an error
+func (A0) m2() {}
+
+// Type aliases and the original type name can be used interchangeably.
+var _ A0 = T0{}
+var _ T0 = A0{}
+
+// But aliases and original types cannot be used with new types based on them.
+var _ N0 = T0{} // ERROR "cannot use T0 literal \(type T0\) as type N0 in assignment"
+var _ N0 = A0{} // ERROR "cannot use T0 literal \(type T0\) as type N0 in assignment"
+
+var _ A5 = Value{}
+
+var _ interface {
+	m1()
+	m2()
+} = T0{}
+
+var _ interface {
+	m1()
+	m2()
+} = A0{}
+
 func _() {
-	type _ = int           // ERROR "unimplemented"
-	type _ = struct{}      // ERROR "unimplemented"
-	type _ = reflect.Value // ERROR "unimplemented"
-	type _ = Value         // ERROR "unimplemented"
+	type _ = T0
+	type _ = int
+	type _ = struct{}
+	type _ = reflect.Value
+	type _ = Value
 
 	type (
-		a1 = int           // ERROR "unimplemented"
-		a2 = struct{}      // ERROR "unimplemented"
-		a3 = reflect.Value // ERROR "unimplemented"
-		a4 = Value         // ERROR "unimplemented"
+		A0 = T0
+		A1 = int
+		A2 = struct{}
+		A3 = reflect.Value
+		A4 = Value
+		A5 Value
+
+		N0 A0
 	)
+
+	var _ A0 = T0{}
+	var _ T0 = A0{}
+
+	var _ N0 = T0{} // ERROR "cannot use T0 literal \(type T0\) as type N0 in assignment"
+	var _ N0 = A0{} // ERROR "cannot use T0 literal \(type T0\) as type N0 in assignment"
+
+	var _ A5 = Value{} // ERROR "cannot use reflect\.Value literal \(type reflect.Value\) as type A5 in assignment"
 }
 
 // Invalid type alias declarations.
 
-type _ = reflect.ValueOf // ERROR "reflect.ValueOf is not a type|unimplemented"
+type _ = reflect.ValueOf // ERROR "reflect.ValueOf is not a type"
+
+func (A1) m() {} // ERROR "cannot define new methods on non-local type int"
+
+type B1 = struct{}
 
-type b1 = struct{} // ERROR "unimplemented"
-func (b1) m()      {} // disabled ERROR "invalid receiver type"
+func (B1) m() {} // ERROR "invalid receiver type"
 
 // TODO(gri) expand
-// It appears that type-checking exits after some more severe errors, so we may
-// need more test files.