Oh Also, @FAKERINHEART, do you have a test case that exercises this change to the for-loop?. That would be nice because then we could add the test case as well for the new lines.

No, I think that it may be no needs to add a new test case. Please look into mpfr.hpp:431, and its commit log: 485fa7c. It was fixed for the mpfr_float_imp<digits10, allocate_dynamic> three years ago, but it forgot to fix mpfr_float_imp<digits10, allocate_stack> which is for this merge case. They are in the same condition, and the previous has been add tests in the 485fa7c.