Create index.md

Author: Soupstraw

message-index/messages/GHC-58008/index.md

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+---
+title: Pattern matching on GADTs without MonoLocalBinds is fragile
+summary: Pattern matching on GADTs without MonoLocalBinds is fragile
+introduced: 9.6.1
+severity: warning
+---
+
+This warning is triggered on pattern matching involving GADTs, if `MonoLocalBinds` is disabled. Type inference can be fragile in this case.
+See the OutsideIn(X) paper and Let-generalisation for more details.
+To resolve this warning, you can enable `MonoLocalBinds` or an extension implying it (`GADTs` or `TypeFamilies`).
+The warning is also triggered when matching on GADT-like pattern synonyms (i.e. pattern synonyms containing equalities in provided constraints).
+In previous versions of GHC (9.2 and below), it was an error to pattern match on a GADT if neither `GADTs` nor `TypeFamilies` were enabled.