evaluator: funcCast set value directly without types.DataItem.

Other expression check `expr.GetValue() == nil` returns false when we wrap nil in DataItem.

optimizer/evaluator/evaluator.go

@@ -627,20 +627,18 @@ func (e *Evaluator) funcConvert(f *ast.FuncConvertExpr) bool {
 
 func (e *Evaluator) funcCast(v *ast.FuncCastExpr) bool {
 	value := v.Expr.GetValue()
-	d := &types.DataItem{Type: v.Tp}
 	// Casting nil to any type returns null
 	if value == nil {
-		d.Data = nil
-		v.SetValue(d)
+		v.SetValue(nil)
 		return true
 	}
 	var err error
-	d.Data, err = types.Cast(value, v.Tp)
+	value, err = types.Cast(value, v.Tp)
 	if err != nil {
 		e.err = errors.Trace(err)
 		return false
 	}
-	v.SetValue(d.Data)
+	v.SetValue(value)
 	return true
 }
 

optimizer/evaluator/evaluator_test.go

@@ -23,6 +23,7 @@ import (
 	"github.com/pingcap/tidb/mysql"
 	"github.com/pingcap/tidb/parser"
 	"github.com/pingcap/tidb/parser/opcode"
+	"github.com/pingcap/tidb/util/charset"
 	"github.com/pingcap/tidb/util/mock"
 	"github.com/pingcap/tidb/util/types"
 )
@@ -383,6 +384,41 @@ func (s *testEvaluatorSuite) TestConvert(c *C) {
 	}
 }
 
+func (s *testEvaluatorSuite) TestCast(c *C) {
+	f := types.NewFieldType(mysql.TypeLonglong)
+
+	expr := &ast.FuncCastExpr{
+		Expr: ast.NewValueExpr(1),
+		Tp:   f,
+	}
+	ctx := mock.NewContext()
+	v, err := Eval(ctx, expr)
+	c.Assert(err, IsNil)
+	c.Assert(v, Equals, int64(1))
+
+	f.Flag |= mysql.UnsignedFlag
+	v, err = Eval(ctx, expr)
+	c.Assert(err, IsNil)
+	c.Assert(v, Equals, uint64(1))
+
+	f.Tp = mysql.TypeString
+	f.Charset = charset.CharsetBin
+	v, err = Eval(ctx, expr)
+	c.Assert(err, IsNil)
+	c.Assert(v, DeepEquals, []byte("1"))
+
+	f.Tp = mysql.TypeString
+	f.Charset = "utf8"
+	v, err = Eval(ctx, expr)
+	c.Assert(err, IsNil)
+	c.Assert(v, DeepEquals, "1")
+
+	expr.Expr = ast.NewValueExpr(nil)
+	v, err = Eval(ctx, expr)
+	c.Assert(err, IsNil)
+	c.Assert(v, IsNil)
+}
+
 func (s *testEvaluatorSuite) TestDateArith(c *C) {
 	c.Skip("to be implement")
 	ctx := mock.NewContext()