[mpich-discuss] MPI_Accumulate with MAXLOC does not error for incompatible types

[Jed Brown]

With MPI_Reduce, we get a nice helpful error message:

Fatal error in PMPI_Reduce: Invalid MPI_Op, error stack:
PMPI_Reduce(1217)...........: MPI_Reduce(sbuf=0x7fff3e39fa50,
rbuf=0x7fff3e39fa40, count=1, dtype=USER<contig>, MPI_MAXLOC, root=0,
MPI_COMM_WORLD) failed
MPIR_MAXLOC_check_dtype(151): MPI_Op MPI_MAXLOC operation not defined for
this datatype


With MPI_Accumulate, we just get wrong results. See attached code using a
user-defined contiguous results.

I care about this because I'm stuck [1] dispatching into alternative
mechanisms for type combinations that are not directly supported by MPI
(e.g., struct {int u; long long k;} and __float128). I missed a spot and
had a confusing bug that I think MPICH should be catching.


[1] As usual, the crud would evaporate if only MPI_Accumulate were
extensible over datatypes.
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