cgen: fix codegen for alias for array fixed initialization (fix #25512) (#25540)

Author: felipensp

vlib/v/gen/c/array.v

@@ -134,8 +134,8 @@ fn (mut g Gen) fixed_array_init(node ast.ArrayInit, array_type Type, var_name st
 	}
 	is_none := node.is_option && !node.has_init && !node.has_val
 
-	if (g.inside_struct_init && g.inside_cast && !g.inside_memset && !g.inside_opt_or_res)
-		|| (node.is_option && !is_none) {
+	if (g.inside_struct_init && g.inside_cast && !g.inside_memset && !g.inside_opt_or_res
+		&& !g.inside_sumtype_cast) || (node.is_option && !is_none) {
 		ret_typ_str := g.styp(node.typ)
 		g.write('(${ret_typ_str})')
 	}

vlib/v/gen/c/cgen.v

@@ -154,6 +154,7 @@ mut:
 	inside_for_c_stmt         bool
 	inside_cast_in_heap       int // inside cast to interface type in heap (resolve recursive calls)
 	inside_cast               bool
+	inside_sumtype_cast       bool
 	inside_selector           bool
 	inside_selector_deref     bool // indicates if the inside selector was already dereferenced
 	inside_memset             bool
@@ -2968,10 +2969,13 @@ fn (mut g Gen) call_cfn_for_casting_expr(fname string, expr ast.Expr, exp ast.Ty
 			ast.void_type)
 		g.write(g.type_default(ctyp))
 	} else {
+		old_inside_sumtype_cast := g.inside_sumtype_cast
+		g.inside_sumtype_cast = true
 		old_left_is_opt := g.left_is_opt
 		g.left_is_opt = !exp.has_flag(.option)
 		g.expr(expr)
 		g.left_is_opt = old_left_is_opt
+		g.inside_sumtype_cast = old_inside_sumtype_cast
 	}
 	if is_sumtype_cast {
 		// the `_to_sumtype_` family of functions last `is_mut` param

vlib/v/tests/casts/cast_sumtype_fixed_array_test.v

@@ -0,0 +1,21 @@
+module main
+
+struct Ipv4 {
+	address [4]u8
+}
+
+struct Ipv6 {
+	address [16]u8
+}
+
+type Address = Ipv4 | Ipv6
+
+fn test_main() {
+	address := Address(Ipv4{
+		address: [u8(192), 168, 1, 1]!
+	})
+
+	assert address == Address(Ipv4{
+		address: [u8(192), 168, 1, 1]!
+	})
+}