checker: require else branch in [flag] enum match (#19375)

vlib/v/checker/match.v

@@ -396,6 +396,9 @@ fn (mut c Checker) match_exprs(mut node ast.MatchExpr, cond_type_sym ast.TypeSym
 						unhandled << '`.${v}`'
 					}
 				}
+				if cond_type_sym.info.is_flag {
+					is_exhaustive = false
+				}
 			}
 			else {
 				is_exhaustive = false

vlib/v/checker/tests/flag_enum_exhaustive_else_branch.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/flag_enum_exhaustive_else_branch.vv:11:1: error: match must be exhaustive (add `else {}` at the end)
+    9 | dump(perm)
+   10 | 
+   11 | match perm {
+      | ~~~~~~~~~~~~
+   12 |     .read { println('r') }
+   13 |     .write { println('w') }

vlib/v/checker/tests/flag_enum_exhaustive_else_branch.vv

@@ -0,0 +1,16 @@
+[flag]
+enum Permission {
+	read
+	write
+	execute
+}
+
+perm := Permission.read | Permission.write | Permission.execute
+dump(perm)
+
+match perm {
+	.read { println('r') }
+	.write { println('w') }
+	.execute { println('x') }
+	// must require an `else` here
+}