Simon Marlow authored Nov 15, 2018 and Ömer Sinan Ağacan committed Nov 15, 2018

Summary:
The logic in `Note [recursive SRTs]` was correct. However, my
implementation of it wasn't: I got the associativity of
`Set.difference` wrong, which led to an extremely subtle and difficult
to find bug.

Fortunately now we have a test case. I was able to cut down the code
to something manageable, and I've added it to the test suite.

Test Plan:
Before (using my stage 1 compiler without the fix):

```
====> T15892(normal) 1 of 1 [0, 0, 0]
cd "T15892.run" &&  "/home/smarlow/ghc/inplace/bin/ghc-stage1" -o T15892
T15892.hs -dcore-lint -dcmm-lint -no-user-package-db -rtsopts
-fno-warn-missed-specialisations -fshow-warning-groups
-fdiagnostics-color=never -fno-diagnostics-show-caret -Werror=compat
-dno-debug-output  -O
cd "T15892.run" && ./T15892  +RTS -G1 -A32k -RTS
Wrong exit code for T15892(normal)(expected 0 , actual 134 )
Stderr ( T15892 ):
T15892: internal error: evacuate: strange closure type 0
    (GHC version 8.7.20181113 for x86_64_unknown_linux)
    Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
Aborted (core dumped)
*** unexpected failure for T15892(normal)
=====> T15892(g1) 1 of 1 [0, 1, 0]
cd "T15892.run" &&  "/home/smarlow/ghc/inplace/bin/ghc-stage1" -o T15892
T15892.hs -dcore-lint -dcmm-lint -no-user-package-db -rtsopts
-fno-warn-missed-specialisations -fshow-warning-groups
-fdiagnostics-color=never -fno-diagnostics-show-caret -Werror=compat
-dno-debug-output  -O
cd "T15892.run" && ./T15892 +RTS -G1 -RTS +RTS -G1 -A32k -RTS
Wrong exit code for T15892(g1)(expected 0 , actual 134 )
Stderr ( T15892 ):
T15892: internal error: evacuate: strange closure type 0
    (GHC version 8.7.20181113 for x86_64_unknown_linux)
    Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
Aborted (core dumped)
```

After (using my stage 2 compiler with the fix):

```
=====> T15892(normal) 1 of 1 [0, 0, 0]
cd "T15892.run" &&  "/home/smarlow/ghc/inplace/test   spaces/ghc-stage2"
-o T15892 T15892.hs -dcore-lint -dcmm-lint -no-user-package-db -rtsopts
-fno-warn-missed-specialisations -fshow-warning-groups
-fdiagnostics-color=never -fno-diagnostics-show-caret -Werror=compat
-dno-debug-output
cd "T15892.run" && ./T15892  +RTS -G1 -A32k -RTS
=====> T15892(g1) 1 of 1 [0, 0, 0]
cd "T15892.run" &&  "/home/smarlow/ghc/inplace/test   spaces/ghc-stage2"
-o T15892 T15892.hs -dcore-lint -dcmm-lint -no-user-package-db -rtsopts
-fno-warn-missed-specialisations -fshow-warning-groups
-fdiagnostics-color=never -fno-diagnostics-show-caret -Werror=compat
-dno-debug-output
cd "T15892.run" && ./T15892 +RTS -G1 -RTS +RTS -G1 -A32k -RTS
```

Reviewers: bgamari, osa1, erikd

Reviewed By: osa1

Subscribers: rwbarton, carter

GHC Trac Issues: #15892

Differential Revision: https://phabricator.haskell.org/D5334