Build Catch3 as C++17

C++17 is still the widest common target, and a library built on it stays
usable from C++17 and every later standard, while C++20 would have left
out many users.

Concepts become enable_if on named traits in Detail/TypeTraits.hpp, which
also replaces std::remove_cvref_t and std::identity. std::span becomes
std::vector references, std::bit_cast becomes static_cast and memcpy,
template lambdas become helper templates, and std::erase_if, contains,
starts_with and ends_with have their C++17 forms. Random::Between no
longer uses a lambda over a constexpr local, which MSVC rejects outside
/permissive-, so the headers work under its default mode too.

Tests give up designated initializers, which MSVC refuses in C++17. CI
builds the matrix in C++17 and C++20, since the headers promise both.

3 files changed, 15 insertions, 12 deletions

diff --git a/src/Progmasoft/Catch3/Generators/Element.hpp b/src/Progmasoft/Catch3/Generators/Element.hpp
index 087b6355..2c270d18 100644
--- a/src/Progmasoft/Catch3/Generators/Element.hpp
+++ b/src/Progmasoft/Catch3/Generators/Element.hpp
@@ -2,9 +2,9 @@
 // SPDX-License-Identifier: MPL-2.0 WITH AdditionRef-Progmasoft-Exception-1.1
 #pragma once
 
+#include <Progmasoft/Catch3/Detail/TypeTraits.hpp>
 #include <Progmasoft/Catch3/Generator.hpp>
 #include <algorithm>
-#include <concepts>
 #include <cstddef>
 #include <initializer_list>
 #include <iterator>
@@ -20,8 +20,9 @@ namespace Progmasoft::Catch3
     /// The list order is the simplicity order: a value shrinks to the values
     /// listed before it, earliest first, so place the simplest case first.
     /// A value that is not in the list has no candidates.
-    template<typename ValueType>
-        requires std::equality_comparable<ValueType>
+    template<typename ValueType,
+             std::enable_if_t<Detail::kIsEqualityComparable<ValueType>, int>
+             = 0>
     [[nodiscard]] Generator<ValueType>
     Element(std::vector<ValueType> values)
     {
@@ -52,8 +53,9 @@ namespace Progmasoft::Catch3
     }
 
     /// Convenience overload for a literal list of values.
-    template<typename ValueType>
-        requires std::equality_comparable<ValueType>
+    template<typename ValueType,
+             std::enable_if_t<Detail::kIsEqualityComparable<ValueType>, int>
+             = 0>
     [[nodiscard]] Generator<ValueType>
     Element(std::initializer_list<ValueType> values)
     {
diff --git a/src/Progmasoft/Catch3/Generators/Integer.hpp b/src/Progmasoft/Catch3/Generators/Integer.hpp
index 45192b3d..3659aa4f 100644
--- a/src/Progmasoft/Catch3/Generators/Integer.hpp
+++ b/src/Progmasoft/Catch3/Generators/Integer.hpp
@@ -2,9 +2,9 @@
 // SPDX-License-Identifier: MPL-2.0 WITH AdditionRef-Progmasoft-Exception-1.1
 #pragma once
 
+#include <Progmasoft/Catch3/Detail/TypeTraits.hpp>
 #include <Progmasoft/Catch3/Generator.hpp>
 #include <algorithm>
-#include <concepts>
 #include <limits>
 #include <type_traits>
 
@@ -13,8 +13,7 @@ namespace Progmasoft::Catch3
     namespace Detail
     {
 
-        template<std::integral IntegerType>
-            requires(!std::same_as<IntegerType, bool>)
+        template<typename IntegerType, Detail::EnableIfInteger<IntegerType> = 0>
         [[nodiscard]] std::vector<IntegerType>
         ShrinkInteger(IntegerType value,
                       IntegerType minimum,
@@ -24,7 +23,9 @@ namespace Progmasoft::Catch3
             const auto addCandidate = [&](IntegerType candidate) {
                 if (candidate < minimum || candidate > maximum
                     || candidate == value
-                    || std::ranges::find(candidates, candidate)
+                    || std::find(candidates.begin(),
+                                 candidates.end(),
+                                 candidate)
                            != candidates.end())
                 {
                     return;
@@ -86,8 +87,7 @@ namespace Progmasoft::Catch3
 
     /// Generates integers over the inclusive range and shrinks toward simple
     /// values.
-    template<std::integral IntegerType>
-        requires(!std::same_as<IntegerType, bool>)
+    template<typename IntegerType, Detail::EnableIfInteger<IntegerType> = 0>
     [[nodiscard]] Generator<IntegerType>
     Integer(IntegerType minimum = std::numeric_limits<IntegerType>::lowest(),
             IntegerType maximum = std::numeric_limits<IntegerType>::max())
diff --git a/src/Progmasoft/Catch3/Generators/Tuple.hpp b/src/Progmasoft/Catch3/Generators/Tuple.hpp
index df58da05..3e889307 100644
--- a/src/Progmasoft/Catch3/Generators/Tuple.hpp
+++ b/src/Progmasoft/Catch3/Generators/Tuple.hpp
@@ -54,10 +54,11 @@ namespace Progmasoft::Catch3
     /// runner's greedy restart this minimizes every component without
     /// trying the cross product of candidates.
     template<typename... ValueTypes>
-        requires(sizeof...(ValueTypes) > 0)
     [[nodiscard]] Generator<std::tuple<ValueTypes...>>
     Tuple(Generator<ValueTypes>... generators)
     {
+        static_assert(sizeof...(ValueTypes) > 0,
+                      "Tuple requires at least one generator");
         using Value = std::tuple<ValueTypes...>;
         const std::tuple<Generator<ValueTypes>...> components{ std::move(
             generators)... };