From c52ed4ce46c121a2151ffbf50a7175b08a3e57d0 Mon Sep 17 00:00:00 2001
From: Ben Deane <ben@elbeno.com>
Date: Fri, 11 Jul 2025 10:24:06 -0600
Subject: [PATCH] :art: Make `cts_t` and `format_result` easier to format

Problem:
- `cts_t` and `format_result` are non-constexpr values that generally may
  contain values that are constexpr-usable. `CX_WRAP` doesn't recognize this and
  wraps them in a capturing lambda as if they are runtime values.

Solution:
- Expose `cx_value_t` members and call operators (like `std::integral_constant`
  has) to make `cts_t` and `format_result` constexpr usable.

Note:
- `format_result` is compile-time usable iff it has no runtime format arguments
  and its string is also compile-time usable.
---
 include/stdx/ct_format.hpp   | 18 ++++++++++++++++++
 include/stdx/ct_string.hpp   |  2 ++
 include/stdx/type_traits.hpp |  1 -
 test/ct_format.cpp           | 12 +++++++++++-
 4 files changed, 31 insertions(+), 2 deletions(-)

diff --git a/include/stdx/ct_format.hpp b/include/stdx/ct_format.hpp
index f67ab7c..6fdd687 100644
--- a/include/stdx/ct_format.hpp
+++ b/include/stdx/ct_format.hpp
@@ -51,6 +51,24 @@ template <typename Str, typename Args> struct format_result {
                                      format_result const &) -> bool = default;
 };
 
+template <typename Str, typename Args>
+    requires(Args::size() == 0 and is_cx_value_v<Str>)
+struct format_result<Str, Args> {
+    CONSTEVAL static auto ct_string_convertible() -> std::true_type;
+
+    [[no_unique_address]] Str str;
+    [[no_unique_address]] Args args{};
+
+    friend constexpr auto operator+(format_result const &fr) { return +fr.str; }
+
+    constexpr auto operator()() const noexcept { return +(*this); }
+    using cx_value_t [[maybe_unused]] = void;
+
+  private:
+    friend constexpr auto operator==(format_result const &,
+                                     format_result const &) -> bool = default;
+};
+
 template <typename Str, typename Args>
 format_result(Str, Args) -> format_result<Str, Args>;
 template <typename Str> format_result(Str) -> format_result<Str, tuple<>>;
diff --git a/include/stdx/ct_string.hpp b/include/stdx/ct_string.hpp
index f3afcac..fe7cb4d 100644
--- a/include/stdx/ct_string.hpp
+++ b/include/stdx/ct_string.hpp
@@ -145,6 +145,8 @@ template <ct_string S> struct cts_t {
 
     CONSTEVAL static auto ct_string_convertible() -> std::true_type;
     friend constexpr auto operator+(cts_t const &) { return value; }
+    constexpr auto operator()() const noexcept { return value; }
+    using cx_value_t [[maybe_unused]] = void;
 };
 
 template <ct_string X, ct_string Y>
diff --git a/include/stdx/type_traits.hpp b/include/stdx/type_traits.hpp
index 56150d5..caa6dcf 100644
--- a/include/stdx/type_traits.hpp
+++ b/include/stdx/type_traits.hpp
@@ -274,6 +274,5 @@ STDX_PRAGMA(diagnostic pop)
 template <typename T, typename = void> constexpr auto is_complete_v = false;
 template <typename T>
 constexpr auto is_complete_v<T, detail::void_v<sizeof(T)>> = true;
-
 } // namespace v1
 } // namespace stdx
diff --git a/test/ct_format.cpp b/test/ct_format.cpp
index f00485a..b8d028b 100644
--- a/test/ct_format.cpp
+++ b/test/ct_format.cpp
@@ -263,11 +263,21 @@ TEST_CASE("FORMAT a type argument", "[ct_format]") {
     STATIC_REQUIRE(STDX_CT_FORMAT("Hello {}", int) == "Hello int"_fmt_res);
 }
 
-TEST_CASE("FORMAT a constexpr string argument", "[ct_format]") {
+TEST_CASE("FORMAT a constexpr ct_string argument", "[ct_format]") {
     constexpr static auto S = "world"_cts;
     STATIC_REQUIRE(STDX_CT_FORMAT("Hello {}", S) == "Hello world"_fmt_res);
 }
 
+TEST_CASE("FORMAT a cts_t argument", "[ct_format]") {
+    auto S = "world"_ctst;
+    STATIC_REQUIRE(STDX_CT_FORMAT("Hello {}", S) == "Hello world"_fmt_res);
+}
+
+TEST_CASE("FORMAT a format_result argument", "[ct_format]") {
+    auto S = "world"_fmt_res;
+    STATIC_REQUIRE(STDX_CT_FORMAT("Hello {}", S) == "Hello world"_fmt_res);
+}
+
 TEST_CASE("FORMAT a constexpr int argument", "[ct_format]") {
     constexpr static auto I = 17;
     STATIC_REQUIRE(STDX_CT_FORMAT("Hello {}", I) == "Hello 17"_fmt_res);