Have the random generators use the global rng instance

This means that if you nest multiple random generators inside one
test case, they will not return the same sequence of numbers.

Idea taken from #1736 by Amit Herman.

Closes #1736
Closes #1734

projects/SelfTest/IntrospectiveTests/GeneratorsImpl.tests.cpp

@@ -250,7 +250,7 @@ int const& TestGen::get() const {
 
 }
 
-TEST_CASE("GENERATE capture macros", "[generators][internals][.approvals]") {
+TEST_CASE("GENERATE capture macros", "[generators][internals][approvals]") {
     auto value = GENERATE(take(10, random(0, 10)));
 
     non_copyable nc; nc.value = value;
@@ -258,3 +258,28 @@ TEST_CASE("GENERATE capture macros", "[generators][internals][.approvals]") {
     auto value2 = GENERATE_REF(Catch::Generators::GeneratorWrapper<int>(std::unique_ptr<Catch::Generators::IGenerator<int>>(new TestGen(nc))));
     REQUIRE(value == value2);
 }
+
+TEST_CASE("Multiple random generators in one test case output different values", "[generators][internals][approvals]") {
+    SECTION("Integer") {
+        auto random1 = Catch::Generators::random(0, 1000);
+        auto random2 = Catch::Generators::random(0, 1000);
+        size_t same = 0;
+        for (size_t i = 0; i < 1000; ++i) {
+            same += random1.get() == random2.get();
+            random1.next(); random2.next();
+        }
+        // 0.5% seems like a sane bound for random identical elements within 1000 runs
+        REQUIRE(same < 5);
+    }
+    SECTION("Float") {
+        auto random1 = Catch::Generators::random(0., 1000.);
+        auto random2 = Catch::Generators::random(0., 1000.);
+        size_t same = 0;
+        for (size_t i = 0; i < 1000; ++i) {
+            same += random1.get() == random2.get();
+            random1.next(); random2.next();
+        }
+        // 0.5% seems like a sane bound for random identical elements within 1000 runs
+        REQUIRE(same < 5);
+    }
+}