The confirmed answer looks correct to me. I think your last comment on it is also correct. The example from the GNU Make manual for SECONDEXPANSION uses patsub

The confirmed answer looks correct to me. I think your last comment on it is also correct. The example from the GNU Make manual for SECONDEXPANSION uses patsubst but is not a pattern rule. You tried to use it with a pattern rule, but then the pattern rule interfered with the % in the patsubst, replacing it before the second expansion, which is when the patsubst ran. All that said... you could probably re-architect your Makefile a bit to make it simpler. I bit more verbosity might be worth it to avoid SECONDEXPANSION tricks.