Commit ce9268f6 authored by simonpj@microsoft.com's avatar simonpj@microsoft.com
Browse files

Do not consult -XGADTs flag when pattern matching on GADTs

See Trac #2004, and Note [Flags and equational constraints] in TcPat.
parent f01efdad
......@@ -867,11 +867,8 @@ refineAlt con pstate ex_tvs [] pat_ty
= return pstate -- Common case: no equational constraints
refineAlt con pstate ex_tvs co_vars pat_ty
= do { opt_gadt <- doptM Opt_GADTs -- No type-refinement unless GADTs are on
; if (not opt_gadt) then return pstate
else do
{ checkTc (isRigidTy pat_ty) (nonRigidMatch con)
= -- See Note [Flags and equational constraints]
do { checkTc (isRigidTy pat_ty) (nonRigidMatch con)
-- We are matching against a GADT constructor with non-trivial
-- constraints, but pattern type is wobbly. For now we fail.
-- We can make sense of this, however:
......@@ -899,9 +896,20 @@ refineAlt con pstate ex_tvs co_vars pat_ty
vcat [ ppr con <+> ppr ex_tvs,
ppr [(v, tyVarKind v) | v <- co_vars],
ppr reft]
} } }
} }
\end{code}
Note [Flags and equational constraints]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
If there are equational constraints, we take account of them
regardless of flag settings; -XGADTs etc applies only to the
*definition* of a data type.
An alternative would be also to reject a program that *used*
constructors with equational constraints. But want we should avoid at
all costs is simply to *ignore* the constraints, since that gives
incomprehensible errors (Trac #2004).
%************************************************************************
%* *
......
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