Support unsigned ints too, add tes for overflow error

Signed-off-by: Juanjo Alvarez <juanjo@sourced.tech>

sql/expression/function/greatest_least.go

@@ -9,6 +9,9 @@ import (
 	"gopkg.in/src-d/go-mysql-server.v0/sql"
 )
 
+var ErrUintOverflow = errors.NewKind(
+	"Unsigned integer too big to fit on signed integer")
+
 // compEval is used to implement Greatest/Least Eval() using a comparison function
 func compEval(
 	returnType sql.Type,
@@ -32,12 +35,35 @@ func compEval(
 		}
 
 		switch t := val.(type) {
-		case int, int32, int64:
+		case int, int8, int16, int32, int64, uint,
+			uint8, uint16, uint32, uint64:
 			switch x := t.(type) {
 			case int:
 				t = int64(x)
+			case int8:
+				t = int64(x)
+			case int16:
+				t = int64(x)
 			case int32:
 				t = int64(x)
+			case uint:
+				i := int64(x)
+				if i < 0 {
+					return nil, ErrUintOverflow.New()
+				}
+				t = i
+			case uint64:
+				i := int64(x)
+				if i < 0 {
+					return nil, ErrUintOverflow.New()
+				}
+				t = i
+			case uint8:
+				t = int64(x)
+			case uint16:
+				t = int64(x)
+			case uint32:
+				t = int64(x)
 			}
 			ival := t.(int64)
 			if i == 0 || cmp(ival, int64(selectedNum)) {

sql/expression/function/greatest_least_test.go

@@ -5,6 +5,7 @@ import (
 	"gopkg.in/src-d/go-mysql-server.v0/sql"
 	"gopkg.in/src-d/go-mysql-server.v0/sql/expression"
 	"testing"
+	"unsafe"
 )
 
 func TestGreatest(t *testing.T) {
@@ -95,6 +96,35 @@ func TestGreatest(t *testing.T) {
 	}
 }
 
+func TestGreatestUnsignedOverflow(t *testing.T) {
+	require := require.New(t)
+
+	var x int
+	var gr sql.Expression
+	var err error
+
+	switch unsafe.Sizeof(x) {
+	case 4:
+		gr, err = NewGreatest(
+			expression.NewLiteral(int32(1), sql.Int32),
+			expression.NewLiteral(uint32(4294967295), sql.Uint32),
+		)
+		require.NoError(err)
+	case 8:
+		gr, err = NewGreatest(
+			expression.NewLiteral(int64(1), sql.Int64),
+			expression.NewLiteral(uint64(18446744073709551615), sql.Uint64),
+		)
+		require.NoError(err)
+	default:
+		// non 32/64 bits??
+		return
+	}
+
+	_, err = gr.Eval(sql.NewEmptyContext(), nil)
+	require.EqualError(err, "Unsigned integer too big to fit on signed integer")
+}
+
 func TestLeast(t *testing.T) {
 	testCases := []struct {
 		name     string