cgen: fix sumtype alias cast to same sumtype (fix #25431) (#25443)

Author: felipensp

vlib/v/gen/c/cgen.v

@@ -5595,6 +5595,10 @@ fn (mut g Gen) cast_expr(node ast.CastExpr) {
 	}
 	node_typ_is_option := node.typ.has_flag(.option)
 	if sym.kind in [.sum_type, .interface] {
+		if g.table.unaliased_type(expr_type) == node_typ {
+			g.expr(node.expr)
+			return
+		}
 		if node_typ_is_option && node.expr is ast.None {
 			g.gen_option_error(node.typ, node.expr)
 		} else if node.expr is ast.Ident && g.comptime.is_comptime_variant_var(node.expr) {

vlib/v/tests/sumtypes/sumtype_cast_alias_test.v

@@ -0,0 +1,18 @@
+struct Number {}
+
+struct String {}
+
+struct Variable {}
+
+struct FunctionCall {}
+
+type Value = Number | String | Variable | FunctionCall
+type Statement = Value
+
+fn test_main() {
+	s := Value(Statement(Value(Number{})))
+	assert '${s}' == 'Value(Number{})'
+
+	s2 := Statement(Value(Number{}))
+	assert '${s2}' == 'Statement(Value(Number{}))'
+}