checker: minor cleanup in cast_expr() (#12954)

vlib/v/checker/checker.v

@@ -3487,10 +3487,8 @@ pub fn (mut c Checker) cast_expr(mut node ast.CastExpr) ast.Type {
 			c.error('cannot convert type `$from_type_sym.name` to `$to_type_sym.name` (alias to `$to_type_sym_final.name`)',
 				node.pos)
 		}
-	} else if to_type_sym.kind == .byte && from_type != ast.voidptr_type
+	} else if to_type_sym.kind == .byte && from_type_sym.kind == .alias
-		&& from_type_sym.kind != .enum_ && !from_type.is_int() && !from_type.is_float()
+		&& from_type_sym_final.kind != .byte && !from_type.is_ptr() {
-		&& from_type != ast.bool_type && !from_type.is_ptr() && from_type_sym.kind == .alias
-		&& from_type_sym_final.name != 'byte' {
 		type_name := c.table.type_to_str(from_type)
 		c.error('cannot cast type `$type_name` to `byte`', node.pos)
 	} else if to_type_sym.kind == .struct_ && !to_type.is_ptr()

vlib/v/checker/tests/cast_alias_to_byte_err.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/cast_alias_to_byte_err.vv:5:7: error: cannot cast type `Foo` to `byte`
+    3 | fn main() {
+    4 |     a := Foo('hello')
+    5 |     b := byte(a)
+      |          ~~~~~~~
+    6 |     println(b)
+    7 | }

vlib/v/checker/tests/cast_alias_to_byte_err.vv

@@ -0,0 +1,7 @@
+type Foo = string
+
+fn main() {
+	a := Foo('hello')
+	b := byte(a)
+	println(b)
+}