Wouldn't it be fair to say that ...d5
would only be a legal move if the en
passant condition was not available?
In fact any 2 square pawn move is only
valid if an en passant possibility is
not exercised. I guess I would argue
that this d5 move is not legal because
of that very possibility, the move does not hold unless white ignores it. I think that the en passant rule means in this position that although black tries to get to d5 the pawn gets chopped at d6 and the black king never really is out of check so try another move.

The contradiction is this:the pawn MUST advance two squares to stop white's check. If the adjacent pawn forbids the two square move-black is mated. If a one square restriction is conditional on an immediate capture (as the rule seems to say),then white is lost.

In short what we have here is a conflict between two opposing ideas:a two square move vs the ability to capture the pawn before it can pass your pawn.

BTW,White can capture en passant while in check--but only if the passing pawn gives direct check.

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In <kevin86>'s position, the pawn really does go to d5. The possibility of an ep capture does not force the pawn to sit on d6 for a move. How can you say that ...d5 is not a legal move? The player picks up the pawn and places it on d5. This blocks the check.

Think of it this way. In this position, is 1...d5 2. Bxc6 legal?

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Now I wonder if a check nullifies
the en passant then in the original
black should win.

Also, the rules for ep are simple. The pawn advances 2 squares. On your very next move, and then only, you are allowed to capture that pawn with a properly placed pawn of your own as if the enemy pawn moved forward only 1 square. You can't capture the pawn ep with a piece, which clearly shows us that the pawn does in fact move forward 2 squares and not one.

In this case, white has a sort of quart-grip with his pawn quartet, ready to clothes-line (to use a wrestling term) the opposing potential peon-theives whom might interrupt a promotion ceremony, by setting free a captured piece.

The reason? Pawns advance (potential for double-step out of the gates, not allowed afterwards), promote (4 choices of a way to remove one replaced piece with another) & capture (either using en passant, when the piece location they capture at needn't always be the one that they land at; or, using regular P capturing, when this is not the case) in different ways than many of the other Soldiers of this Army that have some sort occupational title.

The case above would show the b1-h5 & e8-h5 Diagonals also operating on a sort of Valve opening (to approximate some chess composition terminology, as in the case of the King-Bishop-Pawn running a sort of temporary Interference on a former Diagonal that the Queen might like to retreat on in future), which are in addition to extending a King-Rook & King file for white, along with a 5th Rank, & finally, removal of a counter-attack by another Pawn, such as the King-Knight-Pawn that attempts to attack the Rook.

The previously mentioned rules for Pawn & other piece movement innovations also contributed to the game of chess over the Centuries, as mentioned above.

Cataloguing them by the theme classifications of a, b, c, ..., ... i, j of the Sahovski "Anthology of Chess Combinations" might be a bit more advanced than the more usual tactical methods.