cgen: fix cast option ptr (#17884)

vlib/v/gen/c/assign.v

@@ -56,9 +56,10 @@ fn (mut g Gen) expr_opt_with_cast(expr ast.Expr, expr_typ ast.Type, ret_typ ast.
 	} else {
 		stmt_str := g.go_before_stmt(0).trim_space()
 		styp := g.base_type(ret_typ)
+		decl_styp := g.typ(ret_typ).replace('*', '_ptr')
 		g.empty_line = true
 		tmp_var := g.new_tmp_var()
-		g.writeln('${g.typ(ret_typ)} ${tmp_var};')
+		g.writeln('${decl_styp} ${tmp_var};')
 		g.write('_option_ok(&(${styp}[]) {')
 
 		if expr is ast.CastExpr && expr_typ.has_flag(.option) {

vlib/v/tests/option_ptr_cast_test.v

@@ -0,0 +1,34 @@
+struct ObjectDesc {
+	typ u32
+	ptr voidptr
+}
+
+struct ABCD {
+	name string
+}
+
+pub fn cast_object_desc[H](desc &ObjectDesc) ?H {
+	$if H is &ABCD {
+		/*
+		if desc.typ == 12 { // desc == ABCD
+			return &ABCD(desc.ptr)
+		}*/
+		if desc.typ == 12 { // desc == ABCD
+			return ?&ABCD(&ABCD(desc.ptr))
+		}
+	}
+	return none
+}
+
+fn test_option_ptr() {
+	obj := ABCD{
+		name: 'Foo'
+	}
+	desc := ObjectDesc{
+		typ: 12
+		ptr: voidptr(&obj)
+	}
+	obj2 := cast_object_desc[&ABCD](desc) or { panic('wwww') }
+	// obj2 := &ABCD(desc.ptr)
+	assert obj2.name == 'Foo'
+}