Nil safety checks for cel.Ast (#784)

cel/env.go

@@ -47,16 +47,25 @@ type Ast struct {
 
 // Expr returns the proto serializable instance of the parsed/checked expression.
 func (ast *Ast) Expr() *exprpb.Expr {
+	if ast == nil {
+		return nil
+	}
 	return ast.expr
 }
 
 // IsChecked returns whether the Ast value has been successfully type-checked.
 func (ast *Ast) IsChecked() bool {
+	if ast == nil {
+		return false
+	}
 	return ast.typeMap != nil && len(ast.typeMap) > 0
 }
 
 // SourceInfo returns character offset and newline position information about expression elements.
 func (ast *Ast) SourceInfo() *exprpb.SourceInfo {
+	if ast == nil {
+		return nil
+	}
 	return ast.info
 }
 
@@ -65,9 +74,6 @@ func (ast *Ast) SourceInfo() *exprpb.SourceInfo {
 //
 // Deprecated: use OutputType
 func (ast *Ast) ResultType() *exprpb.Type {
-	if !ast.IsChecked() {
-		return chkdecls.Dyn
-	}
 	out := ast.OutputType()
 	t, err := TypeToExprType(out)
 	if err != nil {
@@ -79,6 +85,9 @@ func (ast *Ast) ResultType() *exprpb.Type {
 // OutputType returns the output type of the expression if the Ast has been type-checked, else
 // returns cel.DynType as the parse step cannot infer types.
 func (ast *Ast) OutputType() *Type {
+	if ast == nil {
+		return types.ErrorType
+	}
 	t, found := ast.typeMap[ast.expr.GetId()]
 	if !found {
 		return DynType
@@ -89,6 +98,9 @@ func (ast *Ast) OutputType() *Type {
 // Source returns a view of the input used to create the Ast. This source may be complete or
 // constructed from the SourceInfo.
 func (ast *Ast) Source() Source {
+	if ast == nil {
+		return nil
+	}
 	return ast.source
 }
 

cel/env_test.go

@@ -29,6 +29,25 @@ import (
 	exprpb "google.golang.org/genproto/googleapis/api/expr/v1alpha1"
 )
 
+func TestAstNil(t *testing.T) {
+	var ast *Ast
+	if ast.IsChecked() {
+		t.Error("ast.IsChecked() returned true for nil ast")
+	}
+	if ast.Expr() != nil {
+		t.Errorf("ast.Expr() got %v, wanted nil", ast.Expr())
+	}
+	if ast.SourceInfo() != nil {
+		t.Errorf("ast.SourceInfo() got %v, wanted nil", ast.SourceInfo())
+	}
+	if ast.OutputType() != types.ErrorType {
+		t.Errorf("ast.OutputType() got %v, wanted error type", ast.OutputType())
+	}
+	if ast.Source() != nil {
+		t.Errorf("ast.Source() got %v, wanted nil", ast.Source())
+	}
+}
+
 func TestIssuesNil(t *testing.T) {
 	var iss *Issues
 	iss = iss.Append(iss)