Weak pointers can cause segfault

The attached program segfaults. It appears to be nondeterministic, but on my machine, it occurs at least 95% of the time. Some small changes, such as removing the bang pattern, cause the segfault to become much less likely.

weakPointerSegfault: internal error: ASSERTION FAILED: file rts/sm/GCAux.c, line 44

    (GHC version 8.0.0.20160411 for x86_64_unknown_linux)
    Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
Aborted (core dumped)

Trac metadata

Trac field	Value
Version	8.0.1-rc3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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