checker: fix invalid cast warning for flag enums (#12848)

vlib/v/checker/checker.v

@@ -3592,24 +3592,30 @@ pub fn (mut c Checker) cast_expr(mut node ast.CastExpr) ast.Type {
 
 			if enum_decl := c.table.enum_decls[to_type_sym.name] {
 				mut in_range := false
-				mut enum_val := 0
+				if enum_decl.is_flag {
+					// if a flag enum has 4 variants, the maximum possible value would have all 4 flags set (0b1111)
+					max_val := (1 << enum_decl.fields.len) - 1
+					in_range = node_val >= 0 && node_val <= max_val
+				} else {
+					mut enum_val := 0
 
-				for enum_field in enum_decl.fields {
-					// check if the field of the enum value is an integer literal
-					if enum_field.expr is ast.IntegerLiteral {
-						enum_val = enum_field.expr.val.int()
+					for enum_field in enum_decl.fields {
+						// check if the field of the enum value is an integer literal
+						if enum_field.expr is ast.IntegerLiteral {
+							enum_val = enum_field.expr.val.int()
+						}
+
+						if node_val == enum_val {
+							in_range = true
+							break
+						}
+
+						enum_val += 1
 					}
-
-					if node_val == enum_val {
-						in_range = true
-						break
-					}
-
-					enum_val += 1
 				}
 
 				if !in_range {
-					c.warn('$node_val does not represents a value of enum $enum_typ_name',
+					c.warn('$node_val does not represent a value of enum $enum_typ_name',
 						node.pos)
 				}
 			}

vlib/v/checker/tests/enum_cast.out

@@ -1,6 +1,27 @@
-vlib/v/checker/tests/enum_cast.vv:6:13: error: 12 does not represents a value of enum Color
-    4 |     println(Color(0))
-    5 |     println(Color(10))
-    6 |     println(Color(12))
+vlib/v/checker/tests/enum_cast.vv:9:13: error: 12 does not represent a value of enum Color
+    7 |     println(Color(0))
+    8 |     println(Color(10))
+    9 |     println(Color(12))
       |             ~~~~~~~~~
-    7 | }
+   10 |     println(Color(-10))
+   11 |
+vlib/v/checker/tests/enum_cast.vv:10:13: error: -10 does not represent a value of enum Color
+    8 |     println(Color(10))
+    9 |     println(Color(12))
+   10 |     println(Color(-10))
+      |             ~~~~~~~~~~
+   11 | 
+   12 |     println(Permissions(0b101))
+vlib/v/checker/tests/enum_cast.vv:13:13: error: 10 does not represent a value of enum Permissions
+   11 | 
+   12 |     println(Permissions(0b101))
+   13 |     println(Permissions(0b1010))
+      |             ~~~~~~~~~~~~~~~~~~~
+   14 |     println(Permissions(-1))
+   15 | }
+vlib/v/checker/tests/enum_cast.vv:14:13: error: -1 does not represent a value of enum Permissions
+   12 |     println(Permissions(0b101))
+   13 |     println(Permissions(0b1010))
+   14 |     println(Permissions(-1))
+      |             ~~~~~~~~~~~~~~~
+   15 | }

vlib/v/checker/tests/enum_cast.vv

@@ -1,7 +1,15 @@
 enum Color { red green = 10 blue }
 
+[flag]
+enum Permissions { read write execute }
+
 fn main() {
     println(Color(0))
     println(Color(10))
     println(Color(12))
-}
+    println(Color(-10))
+
+    println(Permissions(0b101))
+    println(Permissions(0b1010))
+    println(Permissions(-1))
+}