gcc 4.9.2 cygwin64 internal error parsing decimal float point values

[Marc Reynolds]

If the bug is present then it will occur at any optimization level.  

Given this further information I investigated and discovered the problem.  My system somehow had a broken cygmpfr-4.dll installed in /bin.  Given that the assert was from MPFR I should have checked this earlier but I naively assumed that MPFR would have been statically linked.