✨ Add operator+ for format_result conversion to ct_string

Problem:
- It's verbose to explicitly convert a `format_result` to a `ct_string`.
- Docs for `format_result` and `ct_string` are missing some uses, and the use of `ct`.

Solution:
- Add unary `operator+` for that.
- Improve docs for string things.

Author: elbeno

docs/ct_format.adoc

@@ -33,14 +33,20 @@ If there are no runtime arguments, the result is a `format_result` with an empty
 auto s = stdx::ct_format<"Hello {} {}">(CX_VALUE(42), CX_VALUE(17));
 // s is stdx::format_result{"Hello 42 17"_ctst, stdx::tuple{}}
 ----
-...and a `format_result` like this without an runtime arguments is implicitly convertible back to a `ct_string`:
+
+...and a `format_result` like this without an runtime arguments is implicitly convertible back
+to a `ct_string`, or explicitly convertible using unary `operator+`:
 [source,cpp]
 ----
 template <stdx::ct_string S> auto f() { ... };
 
 f<stdx::ct_format<"Hello {}">(CX_VALUE(42))>(); // equivalent to f<"Hello 42">()
++stdx::ct_format<"Hello {}">(CX_VALUE(42));     // equivalent to stdx::ct_string{"Hello 42"}
 ----
 
+NOTE: Using `operator+` on a `format_result` that has a non-empty tuple of
+runtime arguments is a compilation error.
+
 Types and compile-time enumeration values are stringified thus:
 [source,cpp]
 ----

docs/ct_string.adoc

@@ -85,6 +85,8 @@ However, for interfacing with legacy functions, a null terminator can be useful.
 See https://github.com/intel/compile-time-init-build/tree/main/include/sc[cib
 documentation] for details about the cib string constant class.
 
+=== `cts_t`
+
 Sometimes it is useful to wrap a `ct_string` in a type to preserve compile-time
 properties. For this, there is `cts_t` and a corresponding user-defined literal.
 [source,cpp]

docs/utility.adoc

@@ -15,6 +15,34 @@ auto x = 1729;                 // int
 auto y = stdx::as_unsigned(x); // unsigned int
 ----
 
+=== `ct`
+
+`ct` is a function template that produces a
+https://en.cppreference.com/w/cpp/types/integral_constant.html[`std::integral_constant`]
+from a compile-time integral or enumeration type:
+
+[source,cpp]
+----
+constexpr auto x = stdx::ct<42>();      // std::integral_constant<int, 42>{}
+
+enum struct E { A, B, C };
+constexpr auto y = stdx::ct<E::A>();    // std::integral_constant<E, E::A>{}
+----
+
+Or, when given a type, it produces a `stdx::type_identity`:
+
+[source,cpp]
+----
+constexpr auto z = stdx::ct<int>();     // stdx::type_identity<int>{}
+----
+
+Or, when given a string literal, it produces a xref:ct_string.adoc#_cts_t[`stdx::cts_t`]:
+
+[source,cpp]
+----
+constexpr auto s = stdx::ct<"Hello">(); // stdx::cts_t<"Hello">{}
+----
+
 === `CX_VALUE`
 
 `CX_VALUE` is a macro that wraps its argument in a constexpr callable, which can

include/stdx/ct_format.hpp

@@ -32,6 +32,18 @@ template <typename Str, typename Args> struct format_result {
     [[no_unique_address]] Str str;
     [[no_unique_address]] Args args{};
 
+    friend constexpr auto operator+(format_result const &fr)
+        requires(decltype(ct_string_convertible())::value)
+    {
+        return ct_string{fr.str.value};
+    }
+
+    friend constexpr auto operator+(format_result const &) {
+        static_assert(decltype(ct_string_convertible())::value,
+                      "Unary operator+ can only be used on a format_result "
+                      "without any runtime arguments");
+    }
+
   private:
     friend constexpr auto operator==(format_result const &,
                                      format_result const &) -> bool = default;

test/ct_format.cpp

@@ -160,6 +160,12 @@ TEST_CASE("empty format_result can implicitly convert to ct_string",
     STATIC_REQUIRE(conversion_success<stdx::ct_format<"Hello">()>);
 }
 
+TEST_CASE("empty format_result can explicitly convert to ct_string",
+          "[ct_format]") {
+    using namespace std::string_view_literals;
+    STATIC_REQUIRE(+stdx::ct_format<"Hello">() == "Hello"_cts);
+}
+
 namespace {
 template <typename T, T...> struct string_constant {
   private: