From b3283356520766db8a41e1a8aa4b990646c12287 Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Wed, 25 Mar 2026 16:42:19 +0300
Subject: [PATCH] checker: support flag_var bitwise_operator= .enum_flag (fixes
 #20940)

---
 vlib/v/checker/assign.v                       | 15 ++++++++--
 .../enums/enum_flag_compound_assign_test.v    | 28 +++++++++++++++++++
 2 files changed, 41 insertions(+), 2 deletions(-)
 create mode 100644 vlib/v/tests/enums/enum_flag_compound_assign_test.v

diff --git a/vlib/v/checker/assign.v b/vlib/v/checker/assign.v
index ef1f14f13..de3aba4d9 100644
--- a/vlib/v/checker/assign.v
+++ b/vlib/v/checker/assign.v
@@ -789,8 +789,19 @@ or use an explicit `unsafe{ a[..] }`, if you do not want a copy of the slice.',
 						right.pos())
 				}
 			}
-			.and_assign, .or_assign, .xor_assign, .mod_assign, .left_shift_assign,
-			.right_shift_assign {
+			.and_assign, .or_assign, .xor_assign {
+				left_final_sym := c.table.final_sym(left_type_unwrapped)
+				if left_final_sym.info is ast.Enum && left_final_sym.info.is_flag {
+					// `@[flag]` tagged enums support compound bitwise assignment.
+				} else if !left_sym.is_int() && !left_final_sym.is_int() {
+					c.error('operator ${node.op.str()} not defined on left operand type `${left_sym.name}`',
+						left.pos())
+				} else if !right_sym.is_int() && !c.table.final_sym(right_type_unwrapped).is_int() {
+					c.error('operator ${node.op.str()} not defined on right operand type `${right_sym.name}`',
+						right.pos())
+				}
+			}
+			.mod_assign, .left_shift_assign, .right_shift_assign {
 				if !left_sym.is_int() && !c.table.final_sym(left_type_unwrapped).is_int() {
 					c.error('operator ${node.op.str()} not defined on left operand type `${left_sym.name}`',
 						left.pos())
diff --git a/vlib/v/tests/enums/enum_flag_compound_assign_test.v b/vlib/v/tests/enums/enum_flag_compound_assign_test.v
new file mode 100644
index 000000000..5574daa21
--- /dev/null
+++ b/vlib/v/tests/enums/enum_flag_compound_assign_test.v
@@ -0,0 +1,28 @@
+@[flag]
+enum FlagEnumCompoundAssign {
+	a
+	b
+	c
+}
+
+type FlagEnumCompoundAssignAlias = FlagEnumCompoundAssign
+
+fn test_flag_enum_compound_assign_with_short_enum_values() {
+	mut flag := FlagEnumCompoundAssign.a
+	flag |= .b
+	assert flag == .a | .b
+	flag ^= .a
+	assert flag == .b
+	flag &= .b
+	assert flag == .b
+}
+
+fn test_flag_enum_alias_compound_assign_with_short_enum_values() {
+	mut flag := FlagEnumCompoundAssignAlias(FlagEnumCompoundAssign.a)
+	flag |= .c
+	assert flag == .a | .c
+	flag ^= .a
+	assert flag == .c
+	flag &= .c
+	assert flag == .c
+}
-- 
2.39.5