From 0dfaeb66fb8457e7339abbd44d5c53a81ad8ae3a Mon Sep 17 00:00:00 2001
From: Simon Peyton Jones <simon.peytonjones@gmail.com>
Date: Mon, 7 Oct 2024 15:56:12 +0100
Subject: [PATCH] Wibbles
---
compiler/GHC/Tc/Errors.hs | 2 ++
compiler/GHC/Tc/Types/Constraint.hs | 14 +++++++-------
2 files changed, 9 insertions(+), 7 deletions(-)
diff --git a/compiler/GHC/Tc/Errors.hs b/compiler/GHC/Tc/Errors.hs
index 8379202af0a..ce325b285db 100644
--- a/compiler/GHC/Tc/Errors.hs
+++ b/compiler/GHC/Tc/Errors.hs
@@ -470,6 +470,8 @@ mkErrorItem ct
flav = ctFlavour ct
; (suppress, m_evdest) <- case ctEvidence ct of
+ -- For this `suppress` stuff
+ -- see Note [Wanteds rewrite Wanteds] in GHC.Tc.Types.Constraint
CtGiven {} -> return (False, Nothing)
CtWanted { ctev_rewriters = rewriters, ctev_dest = dest }
-> do { rewriters' <- zonkRewriterSet rewriters
diff --git a/compiler/GHC/Tc/Types/Constraint.hs b/compiler/GHC/Tc/Types/Constraint.hs
index 6816f80428d..26e0d694fb5 100644
--- a/compiler/GHC/Tc/Types/Constraint.hs
+++ b/compiler/GHC/Tc/Types/Constraint.hs
@@ -1459,13 +1459,13 @@ can't be solved. But not quite all such constraints; see wrinkles.
it looks only for IrredCt; all insoluble constraints oare put into
CIrredCan
-(IW2) We only treat it as insoluble if it has an empty rewriter set.
- Otherwise #25325 happens: a Wanted constraint A that is /not/ insoluble
- rewrites some other Wanted constraint B, so B has A in its rewriter
- set. Now B looks insoluble. The danger is that we'll suppress reporting
- B becuase of its empty rewriter set; and suppress reporting A because
- there is an insoluble B lying around. (This suppression happens in
- GHC.Tc.Errors.) Solution: don't treat B as insoluble.
+(IW2) We only treat it as insoluble if it has an empty rewriter set. (See Note
+ [Wanteds rewrite Wanteds].) Otherwise #25325 happens: a Wanted constraint A
+ that is /not/ insoluble rewrites some other Wanted constraint B, so B has A
+ in its rewriter set. Now B looks insoluble. The danger is that we'll
+ suppress reporting B becuase of its empty rewriter set; and suppress
+ reporting A because there is an insoluble B lying around. (This suppression
+ happens in GHC.Tc.Errors.mkErrorItem.) Solution: don't treat B as insoluble.
(IW3) If the Wanted arises from a Given (how can that happen?), don't
treat it as a Wanted insoluble (obviously).
--
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