Fix invalid overflow check in flang

[serge-sans-paille]

Summary

tatically checking for overflow with

if constexpr (sizeof(std::size_t) <= sizeof(std::int64_t)) {
     return static_cast<std::int64_t>(length);
}

Doesn't work if sizeof(std::size_t) == sizeof(std::int64_t) because std::size_t
is unsigned.

if length == std::numeric_limits<size_t> casting it to int64_t is going to overflow.

This code would be much simpler if returning a uint64_t instead of a signed
value...

[rovka]

Right, good catch, thanks! I think it might be slightly clearer if we actually used std::numeric_limits::max in the comparison, but this also works.

flang/runtime/command.cpp

if (argc > 1) {
     return argc - 1;
   }
   return 0;
 }
 
 // Returns the length of the \p string. Assumes \p string is valid.
 static std::int64_t StringLength(const char *string) {
   std::size_t length{std::strlen(string)};
-  if constexpr (sizeof(std::size_t) <= sizeof(std::int64_t)) {
+  if constexpr (sizeof(std::size_t) < sizeof(std::int64_t)) {
     return static_cast<std::int64_t>(length);
   } else {
     std::size_t max{std::numeric_limits<std::int64_t>::max()};
     return length > max ? 0 // Just fail.
                         : static_cast<std::int64_t>(length);
   }
 }