go/types: local type-checking of alias declarations

Does not handle imports of packages with exported aliases yet.

For #17592.

Change-Id: Iee63fb9d521014995003a417271fbe0384ae04ef
Reviewed-on: https://go-review.googlesource.com/32108
Reviewed-by: Alan Donovan <adonovan@google.com>

src/go/types/call.go

@@ -275,21 +275,24 @@ func (check *Checker) selector(x *operand, e *ast.SelectorExpr) {
 	// so we don't need a "package" mode for operands: package names
 	// can only appear in qualified identifiers which are mapped to
 	// selector expressions.
+	// (see also decl.go: checker.aliasDecl)
+	// TODO(gri) factor this code out and share with checker.aliasDecl
 	if ident, ok := e.X.(*ast.Ident); ok {
 		_, obj := check.scope.LookupParent(ident.Name, check.pos)
-		if pkg, _ := obj.(*PkgName); pkg != nil {
-			assert(pkg.pkg == check.pkg)
-			check.recordUse(ident, pkg)
-			pkg.used = true
-			exp := pkg.imported.scope.Lookup(sel)
+		if pname, _ := obj.(*PkgName); pname != nil {
+			assert(pname.pkg == check.pkg)
+			check.recordUse(ident, pname)
+			pname.used = true
+			pkg := pname.imported
+			exp := pkg.scope.Lookup(sel)
 			if exp == nil {
-				if !pkg.imported.fake {
-					check.errorf(e.Pos(), "%s not declared by package %s", sel, ident)
+				if !pkg.fake {
+					check.errorf(e.Pos(), "%s not declared by package %s", sel, pkg.name)
 				}
 				goto Error
 			}
 			if !exp.Exported() {
-				check.errorf(e.Pos(), "%s not exported by package %s", sel, ident)
+				check.errorf(e.Pos(), "%s not exported by package %s", sel, pkg.name)
 				// ok to continue
 			}
 			check.recordUse(e.Sel, exp)

src/go/types/decl.go

@@ -85,6 +85,9 @@ func (check *Checker) objDecl(obj Object, def *Named, path []*TypeName) {
 	case *Func:
 		// functions may be recursive - no need to track dependencies
 		check.funcDecl(obj, d)
+	case *Alias:
+		// aliases cannot be recursive - no need to track dependencies
+		check.aliasDecl(obj, d)
 	default:
 		unreachable()
 	}
@@ -329,6 +332,85 @@ func (check *Checker) funcDecl(obj *Func, decl *declInfo) {
 	}
 }
 
+func (check *Checker) aliasDecl(obj *Alias, decl *declInfo) {
+	assert(obj.typ == nil)
+
+	// alias declarations cannot use iota
+	assert(check.iota == nil)
+
+	// assume alias is invalid to start with
+	obj.typ = Typ[Invalid]
+
+	// rhs must be package-qualified identifer pkg.sel (see also call.go: checker.selector)
+	// TODO(gri) factor this code out and share with checker.selector
+	rhs := decl.init
+	var pkg *Package
+	var sel *ast.Ident
+	if sexpr, ok := rhs.(*ast.SelectorExpr); ok {
+		if ident, ok := sexpr.X.(*ast.Ident); ok {
+			_, obj := check.scope.LookupParent(ident.Name, check.pos)
+			if pname, _ := obj.(*PkgName); pname != nil {
+				assert(pname.pkg == check.pkg)
+				check.recordUse(ident, pname)
+				pname.used = true
+				pkg = pname.imported
+				sel = sexpr.Sel
+			}
+		}
+	}
+	if pkg == nil {
+		check.errorf(rhs.Pos(), "invalid alias: %v is not a package-qualified identifier", rhs)
+		return
+	}
+
+	// qualified identifier must denote an exported object
+	orig := pkg.scope.Lookup(sel.Name)
+	if orig == nil || !orig.Exported() {
+		if !pkg.fake {
+			check.errorf(rhs.Pos(), "%s is not exported by package %s", sel.Name, pkg.name)
+		}
+		return
+	}
+	check.recordUse(sel, orig)
+
+	// An alias declaration must not refer to package unsafe.
+	if orig.Pkg() == Unsafe {
+		check.errorf(rhs.Pos(), "invalid alias: %s refers to package unsafe (%v)", obj.Name(), orig)
+		return
+	}
+
+	// The original must be of the same kind as the alias declaration.
+	var why string
+	switch obj.kind {
+	case token.CONST:
+		if _, ok := orig.(*Const); !ok {
+			why = "constant"
+		}
+	case token.TYPE:
+		if _, ok := orig.(*TypeName); !ok {
+			why = "type"
+		}
+	case token.VAR:
+		if _, ok := orig.(*Var); !ok {
+			why = "variable"
+		}
+	case token.FUNC:
+		if _, ok := orig.(*Func); !ok {
+			why = "function"
+		}
+	default:
+		unreachable()
+	}
+	if why != "" {
+		check.errorf(rhs.Pos(), "invalid alias: %v is not a %s", orig, why)
+		return
+	}
+
+	// alias is valid
+	obj.typ = orig.Type()
+	obj.orig = orig
+}
+
 func (check *Checker) declStmt(decl ast.Decl) {
 	pkg := check.pkg
 

src/go/types/object.go

@@ -152,7 +152,6 @@ func NewConst(pos token.Pos, pkg *Package, name string, typ Type, val constant.V
 }
 
 func (obj *Const) Val() constant.Value { return obj.val }
-
 func (*Const) isDependency()           {} // a constant may be a dependency of an initialization expression
 
 // A TypeName represents a declared type.
@@ -186,9 +185,7 @@ func NewField(pos token.Pos, pkg *Package, name string, typ Type, anonymous bool
 }
 
 func (obj *Var) Anonymous() bool { return obj.anonymous }
-
 func (obj *Var) IsField() bool   { return obj.isField }
-
 func (*Var) isDependency()       {} // a variable may be a dependency of an initialization expression
 
 // A Func represents a declared function, concrete method, or abstract
@@ -215,11 +212,22 @@ func (obj *Func) FullName() string {
 	return buf.String()
 }
 
-func (obj *Func) Scope() *Scope {
-	return obj.typ.(*Signature).scope
+func (obj *Func) Scope() *Scope { return obj.typ.(*Signature).scope }
+func (*Func) isDependency()     {} // a function may be a dependency of an initialization expression
+
+// An Alias represents a declared alias.
+type Alias struct {
+	object
+	kind token.Token // token.CONST, token.TYPE, token.VAR, or token.FUNC
+	orig Object      // aliased constant, type, variable, or function
 }
 
-func (*Func) isDependency() {} // a function may be a dependency of an initialization expression
+func NewAlias(pos token.Pos, pkg *Package, name string, kind token.Token, orig Object) *Alias {
+	return &Alias{object{pos: pos, pkg: pkg, name: name}, kind, orig}
+}
+
+func (obj *Alias) Kind() token.Token { return obj.kind }
+func (obj *Alias) Orig() Object      { return obj.orig }
 
 // A Label represents a declared label.
 type Label struct {
@@ -279,6 +287,9 @@ func writeObject(buf *bytes.Buffer, obj Object, qf Qualifier) {
 		}
 		return
 
+	case *Alias:
+		buf.WriteString("alias")
+
 	case *Label:
 		buf.WriteString("label")
 		typ = nil

src/go/types/resolver.go

@@ -14,12 +14,12 @@ import (
 	"unicode"
 )
 
-// A declInfo describes a package-level const, type, var, or func declaration.
+// A declInfo describes a package-level const, type, var, func, or alias declaration.
 type declInfo struct {
 	file  *Scope        // scope of file containing this declaration
 	lhs   []*Var        // lhs of n:1 variable declarations, or nil
 	typ   ast.Expr      // type, or nil
-	init  ast.Expr      // init expression, or nil
+	init  ast.Expr      // init/orig expression, or nil
 	fdecl *ast.FuncDecl // func declaration, or nil
 
 	// The deps field tracks initialization expression dependencies.
@@ -275,7 +275,8 @@ func (check *Checker) collectObjects() {
 						}
 
 					case *ast.AliasSpec:
-						check.errorf(s.Name.Pos(), "cannot handle alias declarations yet")
+						obj := NewAlias(s.Name.Pos(), pkg, s.Name.Name, d.Tok, nil)
+						check.declarePkgObj(s.Name, obj, &declInfo{file: fileScope, init: s.Orig})
 
 					case *ast.ValueSpec:
 						switch d.Tok {

src/go/types/stdlib_test.go

@@ -138,7 +138,6 @@ func TestStdTest(t *testing.T) {
 	}
 
 	testTestDir(t, filepath.Join(runtime.GOROOT(), "test"),
-		"alias2.go",      // excluded until we can handle alias declarations
 		"cmplxdivide.go", // also needs file cmplxdivide1.go - ignore
 		"sigchld.go",     // don't work on Windows; testTestDir should consult build tags
 	)

src/go/types/testdata/aliasdecl.src

@@ -4,7 +4,108 @@
 
 package aliasdecl
 
-import "math"
+import (
+	"flag"
+	"fmt" // use at most once (to test "imported but not used" error)
+	"go/build"
+	. "go/build"
+	"io"
+	"math"
+	"unsafe"
+)
 
-const _ = math.Pi
-const c /* ERROR "cannot handle alias declarations yet" */ => math.Pi
+// helper
+var before struct {
+	f int
+}
+
+// aliases must refer to package-qualified identifiers
+type _ => _ /* ERROR "_ is not a package-qualified identifier" */
+type t1 => _ /* ERROR "_ is not a package-qualified identifier" */
+
+const _ => iota /* ERROR "iota is not a package-qualified identifier" */
+type _ => int   /* ERROR "int is not a package-qualified identifier" */
+
+const c => iota /* ERROR "iota is not a package-qualified identifier" */
+type t2 => int   /* ERROR "int is not a package-qualified identifier" */
+
+// dot-imported identifiers are not qualified identifiers
+// TODO(gri) fix error printing - should not print a qualified identifier...
+var _ => Default /* ERROR "Default is not a package-qualified identifier" */
+
+// qualified identifiers must start with a package
+var _ => before /* ERROR "before.f is not a package-qualified identifier" */ .f
+func _ => before /* ERROR "before.f is not a package-qualified identifier" */ .f
+var _ => after /* ERROR "after.m is not a package-qualified identifier" */ .m
+func _ => after /* ERROR "after.m is not a package-qualified identifier" */ .m
+
+var v1 => before /* ERROR "before.f is not a package-qualified identifier" */ .f
+func f1 => before /* ERROR "before.f is not a package-qualified identifier" */ .f
+var v2 => after /* ERROR "after.m is not a package-qualified identifier" */ .m
+func f2 => after /* ERROR "after.m is not a package-qualified identifier" */ .m
+
+// TODO(gri) fix error printing - should print correct qualified identifier...
+var _ => Default /* ERROR "Default.ARCH is not a package-qualified identifier" */ .ARCH
+var _ Context // use dot-imported package go/build
+
+// aliases may not refer to package unsafe
+type ptr => unsafe /* ERROR "refers to package unsafe" */ .Pointer
+func size => unsafe /* ERROR "refers to package unsafe" */ .Sizeof
+
+// aliases must refer to entities of the same kind
+const _ => math.Pi
+const pi => math.Pi
+const pi1 => math /* ERROR "math.Sin.* is not a constant" */ .Sin
+
+type _ => io.Writer
+type writer => io.Writer
+type writer1 => math /* ERROR "math.Sin.* is not a type" */ .Sin
+
+var _ => build.Default
+var def => build.Default
+var def1 => build /* ERROR "build.Import.* is not a variable" */ .Import
+
+func _ => math.Sin
+func sin => math.Sin
+func sin1 => math /* ERROR "math.Pi.* is not a function" */ .Pi
+
+// using an incorrectly declared alias should not lead to more errors
+const _ = pi1
+type _ writer1
+var _ def1 = 0
+var _ = sin1
+
+// aliases may not be called init
+func init /* ERROR "cannot declare init" */ => flag.Parse
+func _ => flag.Parse // use package flag
+
+// alias reference to a package marks package as used
+func _ => fmt.Println
+
+// re-exported aliases
+const Pi => math.Pi
+
+type Writer => io.Writer
+
+var Def => build.Default
+
+func Sin => math.Sin
+
+// const aliases may appear in "iota" context
+// (this verifies a type-checker internal assertion)
+const (
+	_ = iota
+	pi2 => math.Pi
+)
+
+// type aliases denote identical types
+type myPackage => build.Package
+
+var pkg myPackage
+var _ build.Package = pkg   // valid assignment
+var _ *build.Package = &pkg // valid assignment
+
+// helper
+type after struct{}
+
+func (after) m() {}

src/go/types/typexpr.go

@@ -45,6 +45,19 @@ func (check *Checker) ident(x *operand, e *ast.Ident, def *Named, path []*TypeNa
 		delete(check.unusedDotImports[scope], pkg)
 	}
 
+	// An alias stands for the original object; use that one instead.
+	if alias, _ := obj.(*Alias); alias != nil {
+		if typ == Typ[Invalid] {
+			return
+		}
+		obj = alias.orig
+		// Aliases always refer to non-alias originals.
+		if _, ok := obj.(*Alias); ok {
+			panic("original is an alias")
+		}
+		assert(typ == obj.Type())
+	}
+
 	switch obj := obj.(type) {
 	case *PkgName:
 		check.errorf(e.Pos(), "use of package %s not in selector", obj.name)