add bit to unsigned typecheck and script test

Author: elianddb

enginetest/queries/script_queries.go

@@ -120,6 +120,20 @@ type ScriptTestAssertion struct {
 // Unlike other engine tests, ScriptTests must be self-contained. No other tables are created outside the definition of
 // the tests.
 var ScriptTests = []ScriptTest{
+	{
+		// Regression test for https://github.com/dolthub/dolt/issues/9641
+		Name: "bit union regression test dolt#9641",
+		SetUpScript: []string{
+			"CREATE TABLE bit_test_table (id INT PRIMARY KEY, flag BIT(1))",
+			"INSERT INTO bit_test_table VALUES (1, 0), (2, 1)",
+		},
+		Assertions: []ScriptTestAssertion{
+			{
+				Query:    "SELECT flag FROM bit_test_table WHERE id = 1 UNION SELECT NULL as flag",
+				Expected: []sql.Row{{uint64(0)}, {nil}},
+			},
+		},
+	},
 	{
 		Name: "outer join finish unmatched right side",
 		SetUpScript: []string{

sql/types/typecheck.go

@@ -230,7 +230,7 @@ func IsUnsigned(t sql.Type) bool {
 	if svt, ok := t.(sql.SystemVariableType); ok {
 		t = svt.UnderlyingType()
 	}
-	return t == Uint8 || t == Uint16 || t == Uint24 || t == Uint32 || t == Uint64
+	return t == Uint8 || t == Uint16 || t == Uint24 || t == Uint32 || t == Uint64 || IsBit(t)
 }
 
 // IsYear checks if t is a year type.