Matthew Pickering authored 2 years ago and Marge Bot committed 1 year ago

There was an outright bug, which Simon fixed in July 2021, as a little side-fix on a complicated patch:

```
commit 6656f016
Author: Simon Peyton Jones <simonpj@microsoft.com>
Date:   Fri Jul 23 23:57:01 2021 +0100

    A bunch of changes related to eta reduction

    This is a large collection of changes all relating to eta
    reduction, originally triggered by #18993, but there followed
    a long saga.

    Specifics:

...lots of lines omitted...

    Other incidental changes

    * Fix a fairly long-standing outright bug in the ApplyToVal case of
      GHC.Core.Opt.Simplify.mkDupableContWithDmds. I was failing to take the
      tail of 'dmds' in the recursive call, which meant the demands were All
      Wrong.  I have no idea why this has not caused problems before now.
```

Note this "Fix a fairly longstanding outright bug".   This is the specific fix
```
@@ -3552,8 +3556,8 @@ mkDupableContWithDmds env dmds
         --              let a = ...arg...
         --              in [...hole...] a
         -- NB: sc_dup /= OkToDup; that is caught earlier by contIsDupable
-    do  { let (dmd:_) = dmds   -- Never fails
-        ; (floats1, cont') <- mkDupableContWithDmds env dmds cont
+    do  { let (dmd:cont_dmds) = dmds   -- Never fails
+        ; (floats1, cont') <- mkDupableContWithDmds env cont_dmds cont
         ; let env' = env `setInScopeFromF` floats1
         ; (_, se', arg') <- simplArg env' dup se arg
         ; (let_floats2, arg'') <- makeTrivial env NotTopLevel dmd (fsLit "karg") arg'
```
Ticket #23184 is a report of the bug that this diff fixes.