Fixes #19627.

Previously the solver failed with an unhelpful "solver reached too may iterations" error.
With the fix for #21909 in place we no longer have the possibility of generating such an error if we have `-fconstraint-solver-iteration` > `-fgivens-fuel > `-fwanteds-fuel`. This is true by default, and the said fix also gives programmers a knob to control how hard the solver should try before giving up.

This commit adds:
* Reference to ticket #19627 in the Note [Expanding Recursive Superclasses and ExpansionFuel]
* Test `typecheck/should_fail/T19627.hs` for regression purposes

1 files changed, 2 insertions, 2 deletions

diff --git a/compiler/GHC/Tc/Solver.hs b/compiler/GHC/Tc/Solver.hs
index 81fa86aea4..fad6cbae4f 100644
--- a/compiler/GHC/Tc/Solver.hs
+++ b/compiler/GHC/Tc/Solver.hs
@@ -2371,7 +2371,7 @@ any new unifications, and iterate the implications only if so.
 -}
 
 {- Note [Expanding Recursive Superclasses and ExpansionFuel]
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 Consider the class declaration (T21909)
 
     class C [a] => C a where
@@ -2431,7 +2431,7 @@ There are two preconditions for the default fuel values:
 Precondition (1) ensures that we expand givens at least as many times as we expand wanted constraints
 preferably givenFuel > wantedsFuel to avoid issues like T21909 while
 the precondition (2) ensures that we do not reach the solver iteration limit and fail with a
-more meaningful error message
+more meaningful error message (see T19627)
 
 This also applies for quantified constraints; see `-fqcs-fuel` compiler flag and `QCI.qci_pend_sc` field.
 -}