-O2 makes exception disappear

[benjamin_scarlet]

In this example, an exception throw/catch seems not to happen when optimization is turned on. The behavior seems to be quite finicky. Collapsing the code into fewer than the three given modules seems either to make the bug go away or to produce a "internal error: PAP object entered!" message. Other seemingly innocuous changes produce similar results.

On both Linux and MacOS, this program produces "False" with no optimization flags to ghc, or "True" with -O2.

On 6.10.4 on Linux, it produces "False" with or without -O2.

[benjamin_scarlet]

The Bar module has two functions, one to throw an exception and one to catch it. Think "boiled down HUnit": the throw is a boiled down assert, the catch is a boiled down test-runner function.

The Foo module has a function which conditionally uses the throw function with conditionally-chosen data. Think "boiled down test case." With the arguments given, the test should fail.

The Test module contains main, which calls the function in Foo so as to cause an exception, and tries to catch that exception. Think "test driver".

[RichardG]

Ticket #3961 may be related.

[batterseapower]

I've had a bug report about a similar problem with my test-framework package. The reporter found that the problem was caused by by the full-laziness optimisation.

I haven't had time to look at it in detail yet :(

[simonpj]

I suspect this is related to #3959, which I'm working on.

Simon

[igloo]

(In #4241) I suspect this is #3983, but I'll leave it open so we can check and make sure once #3983 is fixed.

[simonpj]

Fixed, by the same patch as #3959.