Bug 1719959 - Better Tag type choice, fixed corresponding test - r=emilio

On some systems, uint_fast8_t may be as big as size_t! So the `static_assert(sizeof(aIndex) < sizeof(size_t))` could fail there. The better test here is to check for the expected type (uint_fast8_t).

Now, since uint_fast8_t can be bigger than 8 bits, we may as well choose it for variant sizes greater than 255, up to UINT_FAST8_MAX.
(The added parentheses help clang-format distinguish '<' for tests vs for templates.)

Differential Revision: https://phabricator.services.mozilla.com/D119574

mfbt/Variant.h

@@ -112,11 +112,12 @@ struct VariantTag {
   static const size_t TypeCount = sizeof...(Ts);
 
  public:
-  using Type = std::conditional_t < TypeCount < 3, bool,
-        std::conditional_t<TypeCount<(1 << 8), uint_fast8_t,
-                                     size_t  // stop caring past a certain
-                                             // point :-)
-                                     >>;
+  using Type = std::conditional_t<
+      (TypeCount <= 2), bool,
+      std::conditional_t<(TypeCount <= size_t(UINT_FAST8_MAX)), uint_fast8_t,
+                         size_t  // stop caring past a certain
+                                 // point :-)
+                         >>;
 };
 
 // TagHelper gets the given sentinel tag value for the given type T. This has to

mfbt/tests/TestVariant.cpp

@@ -806,7 +806,9 @@ static void testMatchingLambdaWithIndex() {
   // would be no way to distinguish how each lambda is actually invoked because
   // there is only one choice of call operator in each overload set.
   auto desc = [](auto aIndex, auto&& a) {
-    static_assert(sizeof(aIndex) < sizeof(size_t), "Expected small index type");
+    static_assert(
+        std::is_same_v<decltype(aIndex), uint_fast8_t>,
+        "Expected a uint_fast8_t index for a Variant with 3 alternatives");
     if constexpr (std::is_same_v<decltype(a), uint8_t&>) {
       MOZ_RELEASE_ASSERT(aIndex == 0);
       return RESULT(U8, ParamLREF, NA, a);