-pedantic-errors is an option to GCC which forces it to complain about every slight non-conformance (the pedantic) and to treat them all as errors (rather than warnings). The effect of this is that GCC will not compile any code which contains anything even vaguely dubious. This is very good to use when writing your own code (it helps cut down on bugs) but horrid to use on existing code (since it shows every minor, unimportant, wrinkle).
See also: -Wall
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