Memory usage exploding for complex pattern matching

[Victor Miraldo (UU)]

Dear ghc-devs,

We are working on a library and came across quite high memory consumption
when compiling some code (generated by TH).

This memory issue seems to come from the exhaustiveness checker as
we found in:
https://ghc.haskell.org/trac/ghc/ticket/11163
https://ghc.haskell.org/trac/ghc/ticket/11195

We came up with a "minimal" example that at least is self contained and
triggers the problem; `runghc Minimal.hs` should finish with about 5GB of RAM.

Our actual non-minimal scenario runs out of memory on a 16GB RAM machine.

We tried with ghc 8.4.2, 8.0.2 and 7.10.3 and ghc-HEAD; all of those showed
a similar problem, although the older ghc's used less memory.

Do you have any suggestions to make our code finish compiling without running
out of memory?

Thanks!!
Victor
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