checker: fix cast to byte in comptime if (#13036)

vlib/v/checker/checker.v

@@ -2629,7 +2629,7 @@ pub fn (mut c Checker) cast_expr(mut node ast.CastExpr) ast.Type {
 	//        node.typ: `Outside`
 	node.expr_type = c.expr(node.expr) // type to be casted
 
-	mut from_type := node.expr_type
+	mut from_type := c.unwrap_generic(node.expr_type)
 	from_sym := c.table.sym(from_type)
 	final_from_sym := c.table.final_sym(from_type)
 

vlib/v/tests/cast_in_comptime_if_test.v

@@ -0,0 +1,18 @@
+fn test_cast_in_comptime_if() {
+	generic_bool(true)
+}
+
+fn generic_bool<T>(val T) {
+	$if T is bool {
+		println(byte(val))
+		assert byte(val) == 1
+
+		println(i8(val))
+		assert i8(val) == 1
+
+		println(i16(val))
+		assert i16(val) == 1
+	} $else {
+		assert false
+	}
+}