checker: prevent enum value duplication check bypass (#10287)

vlib/v/checker/checker.v

@@ -3208,6 +3208,8 @@ pub fn (mut c Checker) enum_decl(decl ast.EnumDecl) {
 				last := seen[seen.len - 1]
 				if last == checker.int_max {
 					c.error('enum value overflows', field.pos)
+				} else if !decl.is_multi_allowed && last + 1 in seen {
+					c.error('enum value `${last + 1}` already exists', field.pos)
 				}
 				seen << last + 1
 			} else {

vlib/v/checker/tests/enum_field_value_duplicate_a.out

@@ -1,11 +1,11 @@
-vlib/v/checker/tests/enum_field_value_duplicate_err.vv:3:10: error: enum value `0` already exists
+vlib/v/checker/tests/enum_field_value_duplicate_a.vv:3:10: error: enum value `0` already exists
     1 | enum Color {
     2 |     red
     3 |     green = 0
       |             ^
     4 |     blue  = 1
     5 |     alpha = 1
-vlib/v/checker/tests/enum_field_value_duplicate_err.vv:5:10: error: enum value `1` already exists
+vlib/v/checker/tests/enum_field_value_duplicate_a.vv:5:10: error: enum value `1` already exists
     3 |     green = 0
     4 |     blue  = 1
     5 |     alpha = 1

vlib/v/checker/tests/enum_field_value_duplicate_b.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/enum_field_value_duplicate_b.vv:4:2: error: enum value `0` already exists
+    2 |     red // 0
+    3 |     green = -1
+    4 |     blue // -1 + 1 = 0
+      |     ~~~~
+    5 | }

vlib/v/checker/tests/enum_field_value_duplicate_b.vv

@@ -0,0 +1,5 @@
+enum Color {
+	red // 0
+	green = -1
+	blue // -1 + 1 = 0
+}