StringToDouble was renamed StringToMReal as part of code cleanup and the addition of several friends:
?Internal`StringTo*
(*Internal`StringToBoolean Internal`StringToMInteger
Internal`StringToMReal Internal`StringToMRational*)
It should be as fast, but now in failure cases it returns a proper Failure object instead of some bizarro-$Failed[_String]. The error checking could be tighter (the new functions are more discriminating), but at least for now it will just truncate when it encounters invalid characters like the old StringToDouble. Note that the M in the names is for "Machine".
There shouldn't be an Internal`StringToDouble, and I don't see it in a clean, standalone kernel. So perhaps some paclet that is multi-version is referencing and creating it.
Internal\StringToMReal` do it for you? – Carl Lange May 21 '21 at 21:02XMPTools`Helpers`Private`stringToReal). BTW it looks likeInternal`StringToDoubleand its counterpart still exist but have had their defs. removed perSystem`Private`HasAnyCodesQ– b3m2a1 May 21 '21 at 21:03Internal\DoubleToString` – Bob Hanlon May 21 '21 at 21:06MRealToString– b3m2a1 May 22 '21 at 23:24