externals: Add catch2 v3.2.1

Merge commit '6879e5bb1c' as 'externals/catch'

externals/catch/examples/300-Gen-OwnGenerator.cpp

@@ -0,0 +1,69 @@
+// 300-Gen-OwnGenerator.cpp
+// Shows how to define a custom generator.
+
+// Specifically we will implement a random number generator for integers
+// It will have infinite capacity and settable lower/upper bound
+
+#include <catch2/catch_test_macros.hpp>
+#include <catch2/generators/catch_generators.hpp>
+#include <catch2/generators/catch_generators_adapters.hpp>
+
+#include <random>
+
+namespace {
+
+// This class shows how to implement a simple generator for Catch tests
+class RandomIntGenerator : public Catch::Generators::IGenerator<int> {
+    std::minstd_rand m_rand;
+    std::uniform_int_distribution<> m_dist;
+    int current_number;
+public:
+
+    RandomIntGenerator(int low, int high):
+        m_rand(std::random_device{}()),
+        m_dist(low, high)
+    {
+        static_cast<void>(next());
+    }
+
+    int const& get() const override;
+    bool next() override {
+        current_number = m_dist(m_rand);
+        return true;
+    }
+};
+
+// Avoids -Wweak-vtables
+int const& RandomIntGenerator::get() const {
+    return current_number;
+}
+
+// This helper function provides a nicer UX when instantiating the generator
+// Notice that it returns an instance of GeneratorWrapper<int>, which
+// is a value-wrapper around std::unique_ptr<IGenerator<int>>.
+Catch::Generators::GeneratorWrapper<int> random(int low, int high) {
+    return Catch::Generators::GeneratorWrapper<int>(
+        new RandomIntGenerator(low, high)
+        // Another possibility:
+        // Catch::Detail::make_unique<RandomIntGenerator>(low, high)
+    );
+}
+
+} // end anonymous namespaces
+
+// The two sections in this test case are equivalent, but the first one
+// is much more readable/nicer to use
+TEST_CASE("Generating random ints", "[example][generator]") {
+    SECTION("Nice UX") {
+        auto i = GENERATE(take(100, random(-100, 100)));
+        REQUIRE(i >= -100);
+        REQUIRE(i <= 100);
+    }
+    SECTION("Creating the random generator directly") {
+        auto i = GENERATE(take(100, GeneratorWrapper<int>(Catch::Detail::make_unique<RandomIntGenerator>(-100, 100))));
+        REQUIRE(i >= -100);
+        REQUIRE(i <= 100);
+    }
+}
+
+// Compiling and running this file will result in 400 successful assertions