[dev.regabi] go/types: type alias decl requires go1.9

This is a port of CL 289570 to go/types. It has some notable differences
with that CL:
 + A new _BadDecl error code is added, to indicate declarations with bad
   syntax.
 + declInfo is updated hold not an 'alias' bool, but an aliasPos
   token.Pos to identify the location of the type aliasing '=' token.
   This allows for error messages to be accurately placed on the '='

For #31793

Change-Id: Ib15969f9cd5be30228b7a4c6406f978d6fc58018
Reviewed-on: https://go-review.googlesource.com/c/go/+/291318
Trust: Robert Findley <rfindley@google.com>
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

4 files changed, 30 insertions, 10 deletions

diff --git a/src/go/types/decl.go b/src/go/types/decl.go
index 571e172351..b861cde496 100644
--- a/src/go/types/decl.go
+++ b/src/go/types/decl.go
@@ -189,7 +189,7 @@ func (check *Checker) objDecl(obj Object, def *Named) {
 		check.varDecl(obj, d.lhs, d.typ, d.init)
 	case *TypeName:
 		// invalid recursive types are detected via path
-		check.typeDecl(obj, d.typ, def, d.alias)
+		check.typeDecl(obj, d.typ, def, d.aliasPos)
 	case *Func:
 		// functions may be recursive - no need to track dependencies
 		check.funcDecl(obj, d)
@@ -234,7 +234,7 @@ func (check *Checker) cycle(obj Object) (isCycle bool) {
 			// this information explicitly in the object.
 			var alias bool
 			if d := check.objMap[obj]; d != nil {
-				alias = d.alias // package-level object
+				alias = d.aliasPos.IsValid() // package-level object
 			} else {
 				alias = obj.IsAlias() // function local object
 			}
@@ -640,14 +640,17 @@ func (n *Named) setUnderlying(typ Type) {
 	}
 }
 
-func (check *Checker) typeDecl(obj *TypeName, typ ast.Expr, def *Named, alias bool) {
+func (check *Checker) typeDecl(obj *TypeName, typ ast.Expr, def *Named, aliasPos token.Pos) {
 	assert(obj.typ == nil)
 
 	check.later(func() {
 		check.validType(obj.typ, nil)
 	})
 
-	if alias {
+	if aliasPos.IsValid() {
+		if !check.allowVersion(obj.pkg, 1, 9) {
+			check.errorf(atPos(aliasPos), _BadDecl, "type aliases requires go1.9 or later")
+		}
 
 		obj.typ = Typ[Invalid]
 		obj.typ = check.typ(typ)
@@ -678,9 +681,12 @@ func (check *Checker) typeDecl(obj *TypeName, typ ast.Expr, def *Named, alias bo
 
 	}
 
+	// TODO(rFindley): move to the callsite, as this is only needed for top-level
+	//                 decls.
 	check.addMethodDecls(obj)
 }
 
+// TODO(rFindley): rename to collectMethods, to be consistent with types2.
 func (check *Checker) addMethodDecls(obj *TypeName) {
 	// get associated methods
 	// (Checker.collectObjects only collects methods with non-blank names;
@@ -691,7 +697,7 @@ func (check *Checker) addMethodDecls(obj *TypeName) {
 		return
 	}
 	delete(check.methods, obj)
-	assert(!check.objMap[obj].alias) // don't use TypeName.IsAlias (requires fully set up object)
+	assert(!check.objMap[obj].aliasPos.IsValid()) // don't use TypeName.IsAlias (requires fully set up object)
 
 	// use an objset to check for name conflicts
 	var mset objset
@@ -864,7 +870,7 @@ func (check *Checker) declStmt(d ast.Decl) {
 			check.declare(check.scope, d.spec.Name, obj, scopePos)
 			// mark and unmark type before calling typeDecl; its type is still nil (see Checker.objDecl)
 			obj.setColor(grey + color(check.push(obj)))
-			check.typeDecl(obj, d.spec.Type, nil, d.spec.Assign.IsValid())
+			check.typeDecl(obj, d.spec.Type, nil, d.spec.Assign)
 			check.pop().setColor(black)
 		default:
 			check.invalidAST(d.node(), "unknown ast.Decl node %T", d.node())
diff --git a/src/go/types/errorcodes.go b/src/go/types/errorcodes.go
index d27abdf4d4..ac28c3bd13 100644
--- a/src/go/types/errorcodes.go
+++ b/src/go/types/errorcodes.go
@@ -1366,4 +1366,7 @@ const (
 	//  	return i
 	//  }
 	_InvalidGo
+
+	// _BadDecl occurs when a declaration has invalid syntax.
+	_BadDecl
 )
diff --git a/src/go/types/resolver.go b/src/go/types/resolver.go
index 47e165db36..e4411592e8 100644
--- a/src/go/types/resolver.go
+++ b/src/go/types/resolver.go
@@ -23,7 +23,7 @@ type declInfo struct {
 	init      ast.Expr      // init/orig expression, or nil
 	inherited bool          // if set, the init expression is inherited from a previous constant declaration
 	fdecl     *ast.FuncDecl // func declaration, or nil
-	alias     bool          // type alias declaration
+	aliasPos  token.Pos     // If valid, the decl is a type alias and aliasPos is the position of '='.
 
 	// The deps field tracks initialization expression dependencies.
 	deps map[Object]bool // lazily initialized
@@ -366,7 +366,7 @@ func (check *Checker) collectObjects() {
 				}
 			case typeDecl:
 				obj := NewTypeName(d.spec.Name.Pos(), pkg, d.spec.Name.Name, nil)
-				check.declarePkgObj(d.spec.Name, obj, &declInfo{file: fileScope, typ: d.spec.Type, alias: d.spec.Assign.IsValid()})
+				check.declarePkgObj(d.spec.Name, obj, &declInfo{file: fileScope, typ: d.spec.Type, aliasPos: d.spec.Assign})
 			case funcDecl:
 				info := &declInfo{file: fileScope, fdecl: d.decl}
 				name := d.decl.Name.Name
@@ -493,7 +493,7 @@ func (check *Checker) resolveBaseTypeName(typ ast.Expr) (ptr bool, base *TypeNam
 		// we're done if tdecl defined tname as a new type
 		// (rather than an alias)
 		tdecl := check.objMap[tname] // must exist for objects in package scope
-		if !tdecl.alias {
+		if !tdecl.aliasPos.IsValid() {
 			return ptr, tname
 		}
 
@@ -534,7 +534,7 @@ func (check *Checker) packageObjects() {
 	// phase 1
 	for _, obj := range objList {
 		// If we have a type alias, collect it for the 2nd phase.
-		if tname, _ := obj.(*TypeName); tname != nil && check.objMap[tname].alias {
+		if tname, _ := obj.(*TypeName); tname != nil && check.objMap[tname].aliasPos.IsValid() {
 			aliasList = append(aliasList, tname)
 			continue
 		}
diff --git a/src/go/types/testdata/go1_8.src b/src/go/types/testdata/go1_8.src
new file mode 100644
index 0000000000..3ead1e981b
--- /dev/null
+++ b/src/go/types/testdata/go1_8.src
@@ -0,0 +1,11 @@
+// Copyright 2021 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.
+
+// Check Go language version-specific errors.
+
+package go1_8 // go1.8
+
+// type alias declarations
+type any = /* ERROR type aliases requires go1.9 or later */ interface{}
+