I commented in the affirmative on your MR, Simon. Should I also re-assign it back to you? https://gitlab.haskell.org/ghc/ghc/merge_requests/1094 On Sun, Jun 2, 2019 at 12:15 PM Nicolas Frisby wrote:

Thank you for confirming, Simon.

I opened https://gitlab.haskell.org/ghc/ghc/issues/16735

On Sun, Jun 2, 2019 at 8:33 AM Simon Peyton Jones wrote:

...

Nick

That looks outright wrong to me. Would you like to make a ticket for it? It should be easy to fix.

I’m surprised it has not caused more trouble before now.

Well spotted and well diagnosed!

Simon

*From:* ghc-devs *On Behalf Of *Nicolas Frisby *Sent:* 01 June 2019 04:24 *To:* ghc-devs@haskell.org *Subject:* should TcCanonical.rewriteEvidence preserve ctev_nosh?

A bit of context and then my question.

https://gitlab.haskell.org/ghc/ghc/blob/master/compiler/typecheck/TcCanonica... https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgitlab.haskell.org%2Fghc%2Fghc%2Fblob%2Fmaster%2Fcompiler%2Ftypecheck%2FTcCanonical.hs%23L2226&data=02%7C01%7Csimonpj%40microsoft.com%7Cd90efcd1da6f4058c00c08d6e640c7c2%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636949563239275987&sdata=F7dgkceI2512TafrGNP38xvWAv2%2F6HKQscRnU%2Bypbf8%3D&reserved=0 declares `rewriteEvidence` with the comment that "rewriteEvidence old_ev new_pred co ... Returns a new_ev : new_pred, with same wanted/given/derived flag as old_ev".

However, the `CtWanted` case just immediately calls `newWanted`, which ignores the incoming `ctev_nosh` flag and always uses `WDeriv`.

``` rewriteEvidence ev@(CtWanted { ctev_dest = dest

, ctev_loc = loc }) new_pred co

= do { mb_new_ev <- newWanted loc new_pred ```

(This is true in 8.6.5 and also right now at 6febc444c0abea6c033174aa0e813c950b9b2877.)

Thus I'm seeing a [W] is become a [WD], which seems to contradict the comment's "same flag" claim.

MY QUESTION ======> Is this "WD unsplitting" the intended behavior? See canonicalization tc-trace below.

If it is intended, I'll probably write a follow-up question asking for more advice. The constraint solver is looping unproductively because my plugin keeps discarding the (re-)emitted [D] shadow as a tautology, then the [W] "unsplits" during canonicalization (even though my plugin didn't touch the [W]), this re-emits the same [D] (via a fundep), and so on.

My continual disclaimer recently is that I'm seeing this behavior with my in-development plugin activated, so it's entirely I'm pushing GHC into unfamiliar waters or even just plain violating some invariants without realizing it. But this trace looks rather isolated from plugins.

Thanks! -Nick

Here's a -ddump-tc-trace of a canonicalization that goes from [W] to [WD] happen (with GHC 8.6.5):

runStage canonicalization { workitem = [W] $dMonadFree_a35V {0}:: MonadFree ((fs_a2UF[sk:1] :- Const e_a2UE[sk:1]) :+ Const e_a2UE[sk:1]) (Free fs_a2UF[sk:1]) (CDictCan) flatten_args { (fs_a2UF[sk:1] :- Const e_a2UE[sk:1]) :+ Const e_a2UE[sk:1] Free fs_a2UF[sk:1] matchFamTcM Matching: (fs_a2UF[sk:1] :- Const e_a2UE[sk:1]) :+ Const e_a2UE[sk:1] Match failed matchFamTcM Matching: fs_a2UF[sk:1] :- Const e_a2UE[sk:1] Match failed Unfilled tyvar fs_a2UF[sk:1] Unfilled tyvar e_a2UE[sk:1] flatten/flat-cache hit :- [* -> *, fs_a2UF[sk:1], Const e_a2UE[sk:1]] s_a35Z[fmv:1] Unfilled tyvar s_a35Z[fmv:1] Unfilled tyvar e_a2UE[sk:1] matchFamTcM Matching: s_a35Z[fmv:1] :+ Const e_a2UE[sk:1] Match failed New coercion hole: co_a365 Emitting new coercion hole {co_a365} :: (s_a35Z[fmv:1] :+ Const e_a2UE[sk:1]) GHC.Prim.~# s_a364[fmv:1] extendFlatCache :+ [* -> *, s_a35Z[fmv:1], Const e_a2UE[sk:1]] [WD] s_a364[fmv:1] flatten/flat-cache miss :+ [* -> *, s_a35Z[fmv:1], Const e_a2UE[sk:1]] s_a364[fmv:1] [WD] hole{co_a365} {0}:: (s_a35Z[fmv:1] :+ Const e_a2UE[sk:1]) GHC.Prim.~# s_a364[fmv:1] Unfilled tyvar fs_a2UF[sk:1] flatten } s_a364[fmv:1] Free fs_a2UF[sk:1] Emitting new wanted $dMonadFree_a366 :: MonadFree s_a364[fmv:1] (Free fs_a2UF[sk:1]) arising from a use of ‘wrap’ at tests/ill-typed/T7.hs:77:14-17 addTcEvBind a2V1 [W] $dMonadFree_a35V = $dMonadFree_a366 `cast` ((MonadFree (Sym {co_a365} ; ((:+) <* -> *>_N (Sym {co_a360}) _N)_N) _N)_R :: MonadFree s_a364[fmv:1] (Free fs_a2UF[sk:1]) ~R# MonadFree ((fs_a2UF[sk:1] :- Const e_a2UE[sk:1]) :+ Const e_a2UE[sk:1]) (Free fs_a2UF[sk:1])) canClass [W] $dMonadFree_a35V {0}:: MonadFree ((fs_a2UF[sk:1] :- Const e_a2UE[sk:1]) :+ Const e_a2UE[sk:1]) (Free fs_a2UF[sk:1]) MonadFree s_a364[fmv:1] (Free fs_a2UF[sk:1]) ContinueWith [WD] $dMonadFree_a366 {0}:: MonadFree s_a364[fmv:1] (Free fs_a2UF[sk:1]) end stage canonicalization }

and later

try_fundeps [WD] $dMonadFree_a36h {0}:: MonadFree s_a36f[fmv:1] (Free fs_a2UF[sk:1]) (CDictCan) emitFunDepDeriveds 1 0 [fs_a2UF[sk:1] ~ s_a36f[fmv:1]] False Emitting new derived equality [D] _ {0}:: fs_a2UF[sk:1] GHC.Prim.~# s_a36f[fmv:1]

and then my plugin discards that [D] as a tautological, and then the [W] gets unsplit and the [D] comes back around etc