Walter, Andrei, and Herb proposed "static if" in N3329 (after it's big success in D) which was ultimately rejected. Ville Voutilainen explicitly revived it in N4461 with changes. Ville then iterated on that proposal a few times (P0128R0 and P0128R1). Finally, Jens Maurer took that and made some syntax changes with commitee input resulting in P0292R1 which was the accepted proposal. "static if" from D absolutely was the inspiration for "constexpr if". Each proposal references the last in a chain going back to Walter, Andrei, and Herb's proposal.