Fix up default fmod to perform post-condition checking.

[jzmaddock]

Fixes https://github.com/boostorg/multiprecision/issues/604.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 94.1%. Comparing base (f0319ec) to head (062503f).

Additional details and impacted files

[](https://app.codecov.io/gh/boostorg/multiprecision/pull/605?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg) ```diff @@ Coverage Diff @@ ## develop #605 +/- ## ========================================= + Coverage 94.1% 94.1% +0.1% ========================================= Files 273 274 +1 Lines 28525 28563 +38 ========================================= + Hits 26834 26874 +40 + Misses 1691 1689 -2 ``` | [Files](https://app.codecov.io/gh/boostorg/multiprecision/pull/605?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg) | Coverage Δ | | |---|---|---| | [...nclude/boost/multiprecision/detail/default\_ops.hpp](https://app.codecov.io/gh/boostorg/multiprecision/pull/605?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg#diff-aW5jbHVkZS9ib29zdC9tdWx0aXByZWNpc2lvbi9kZXRhaWwvZGVmYXVsdF9vcHMuaHBw) | `90.9% <100.0%> (+0.1%)` | :arrow_up: | | [test/git\_issue\_604.cpp](https://app.codecov.io/gh/boostorg/multiprecision/pull/605?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg#diff-dGVzdC9naXRfaXNzdWVfNjA0LmNwcA==) | `100.0% <100.0%> (ø)` | | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/boostorg/multiprecision/pull/605/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg) ------ [Continue to review full report in Codecov by Sentry](https://app.codecov.io/gh/boostorg/multiprecision/pull/605?src=pr&el=continue&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg). > **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg) > `Δ = absolute (impact)`, `ø = not affected`, `? = missing data` > Powered by [Codecov](https://app.codecov.io/gh/boostorg/multiprecision/pull/605?src=pr&el=footer&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg). Last update [f0319ec...062503f](https://app.codecov.io/gh/boostorg/multiprecision/pull/605?src=pr&el=lastupdated&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg).

[jzmaddock]

Failures are unrelated and are being worked on elsewhere... merging.