cgen: fix struct field init with optional fixed array

vlib/v/gen/c/assign.v

@@ -157,6 +157,18 @@ fn (mut g Gen) expr_with_opt(expr ast.Expr, expr_typ ast.Type, ret_typ ast.Type)
 	return ''
 }
 
+fn (mut g Gen) expr_with_fixed_array_opt(expr ast.Expr, expr_typ ast.Type, ret_typ ast.Type) {
+	tmp_var := g.new_tmp_var()
+	stmt_str := g.go_before_last_stmt().trim_space()
+	styp := g.styp(expr_typ)
+	g.empty_line = true
+	g.writeln('${styp} ${tmp_var} = {.state = 0};')
+	g.write('memcpy(${tmp_var}.data, ')
+	g.expr(expr)
+	g.writeln(', sizeof(${g.styp(ret_typ)}));')
+	g.write2(stmt_str, tmp_var)
+}
+
 fn (mut g Gen) assign_stmt(node_ ast.AssignStmt) {
 	mut node := unsafe { node_ }
 	if node.is_static {

vlib/v/gen/c/struct.v

@@ -692,9 +692,15 @@ fn (mut g Gen) struct_init_field(sfield ast.StructInitField, language ast.Langua
 						field_unwrap_sym.info.elem_type, field_unwrap_sym.info.size)
 				}
 				ast.CastExpr, ast.CallExpr {
-					tmp_var := g.expr_with_var(sfield.expr, sfield.expected_type, false)
-					g.fixed_array_var_init(tmp_var, false, field_unwrap_sym.info.elem_type,
-						field_unwrap_sym.info.size)
+					if sfield.expected_type.has_flag(.option) && !field_unwrap_typ.has_flag(.option) {
+						g.expr_with_fixed_array_opt(sfield.expr, sfield.expected_type,
+							field_unwrap_typ)
+					} else {
+						tmp_var := g.expr_with_var(sfield.expr, sfield.expected_type,
+							false)
+						g.fixed_array_var_init(tmp_var, false, field_unwrap_sym.info.elem_type,
+							field_unwrap_sym.info.size)
+					}
 				}
 				ast.ArrayInit {
 					if sfield.expr.has_index {

vlib/v/tests/structs/struct_field_init_with_fixed_array_opt_test.v

@@ -0,0 +1,15 @@
+type Arr = [4]u8
+
+struct Foo {
+	bar int
+	baz ?Arr
+}
+
+fn test_struct_field_init_with_fixed_array_opt() {
+	f := Foo{
+		bar: 1
+		baz: Arr([u8(5), 4, 3, 2]!)
+	}
+	println(f)
+	assert f.baz as Arr == [u8(5), 4, 3, 2]!
+}