[m-rev.] C# bootheck fails due to C# warnings

[Zoltan Somogyi]

I was attempting to test a possible fix for the NumCUs abort,
but the C# bootcheck failes with "building of stage 2 library
not successful". The contents of mer_std.err is attached.

I find this peculiar, for several reasons.

First, a C# bootcheck in the very same workspace worked
fine just a couple of days ago, modulo approx 280 test case
failures. To the best of my knowledge, I have not changed
the setup of the C# compiler in any way since then, though
I may have added a recent Mercury compiler to my path.

Second, every single line in the attached file is a warning.
None are errors. I don't remember whether the C# compiler
has an equivalent of --halt-at-warn, or whether we use it.

Third, every warning is either 168 or 219, and we disable both
in all parts of a C# source file except for the code we generate
for foreign procs. The problem I see with that exception is that
- both 168 and 219 are about a variable being unused;
- they both seem to be reported on the line containing the declaration
  of the affected variable;
- those lines are part of the implementation of the foreign_proc,
  but they are generated by the compiler for input args of the
  foreign_proc;
- we know that many input args, especially those containing
  type_infos, are never used; but
- we restore both these warnings (and some others) around
  *all* components of the implementation of a foreign_proc,
  and not just the non-compiler-generated components.

I intend to fix that last part tomorrow, but does anyone have any
idea about what may have changed to make this a problem *now*
when it hasn't been a problem before?

Zoltan.
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