Fix #13739 - disable some warnings when calling Function.

[Matafou]

Kind: bug fix

Fixes / closes #13739 by disabling warnings when calling Function. It also fixes the warning about non truly recursive function.

The fix is not entirely satisfactory since

1. Function should not build a Fix when the function is not recursive.
2. Function should raise a warning f the user wrote e useless pattern variable. Only auto generated such variable should be innocuous.

This solution is however acceptable IMHO for the moment.

[gares]

The patch looks good to me.
I'd only add a test case based on #13739 in the test-suite/output directory to assert the warnings are not actually fired.

@Matafou do you have the time for this last improvement? Otherwise I can take care of it myself.

[Matafou]

I will do it.

[Zimmi48]

@liyishuai / @Lysxia: when this PR is merged, you should be able to rely on the new CWarnings.with_warn function instead of the ad hoc code for doing the same thing at https://github.com/QuickChick/QuickChick/blob/56452d27dc978b477e1fc9bae598f1e1560839f1/plugin/quickChick.mlg#L147-L151.

[Matafou]

I will do it.

Not yet done, I need to find some time to reproduce the second warning, to come up with an small example triggering it. see #13739.

[gares]

No problem, but we could also be happy with a test for just one of the 3 warnings you disable (it will test that the whole machinery works).

[Matafou]

No problem, but we could also be happy with a test for just one of the 3 warnings you disable (it will test that the whole machinery works).

Did like this and rebased.

[gares]

@coqbot merge now