Dave Kaye authored Apr 15, 2022 and Alexander Semke committed Apr 15, 2022

To generate a move string including a pawn promotion, the Knights code starts out with a generic call to move.cpp to create a string.
An example string returned is:
e7e8=Q
Then the string is modified in xboardprotocol.cpp to remove all capitals:
e7e8=q
This string is then sent to the chess engine.  But the xboard protocol documentation requires the string to be:
e7e8q

Early on in chess engine development there were multiple formats to communicate with chess engines.
Some utilzed an = sign between move and promotion piece.  But the Xboard system slowly became the standard for communication.
Some chess engines will also accept the = sign in Xboard mode to be historically compatable.
Specifically, the following engines will accept the = sign and will run:
GNUchess6, Crafty, Phalanx XXV and Gully2.

Other (usually newer) engines require the correct Xboard protocol. They will stop play with an illegal move response if the move contains the = sign.
Some examples are:
Greko, Dreamer, Sloppy, Beowulf and Prophet4.

With this fix the behavior is more correct and all of the above engines accept the pawn promotion move and continue to play.

BUG: 447665
FIXED-IN: 22.08