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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
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