Remove undefined behaviour (#117)

* Add assertions for cases where the API documents undefined behaviour.
This affects operator->, operator*, and error() on a tl::expected
instance. The performance impact is likely negligible for typical uses.
refs #113

Signed-off-by: Daira Hopwood <daira@jacaranda.org>

* Add tests for assertions.

Signed-off-by: Daira Hopwood <daira@jacaranda.org>

* Add documentation for assertion behaviour.

Signed-off-by: Daira Hopwood <daira@jacaranda.org>

---------

Signed-off-by: Daira Hopwood <daira@jacaranda.org>

tests/assertions.cpp

@@ -0,0 +1,18 @@
+#include <catch2/catch.hpp>
+#include <stdexcept>
+
+#define TL_ASSERT(cond) if (!(cond)) { throw std::runtime_error(std::string("assertion failure")); }
+
+#include <tl/expected.hpp>
+
+TEST_CASE("Assertions", "[assertions]") {
+  tl::expected<int,int> o1 = 42;
+  REQUIRE_THROWS_WITH(o1.error(), "assertion failure");
+
+  tl::expected<int,int> o2 {tl::unexpect, 0};
+  REQUIRE_THROWS_WITH(*o2, "assertion failure");
+
+  struct foo { int bar; };
+  tl::expected<struct foo,int> o3 {tl::unexpect, 0};
+  REQUIRE_THROWS_WITH(o3->bar, "assertion failure");
+}