Split off -Wunused-type-variables from -Wunused-matches

Summary:
Previously, `-Wunused-matches` would fire whenever it detected unused type
variables in a type family or data family instance. This can be annoying for
users who wish to use type variable names as documentation, as being
`-Wall`-compliant would mean that they'd have to prefix many of their type
variable names with underscores, making the documentation harder to read.

To avoid this, a new warning `-Wunused-type-variables` was created that only
encompasses unused variables in family instances. `-Wunused-matches` reverts
back to its role of only warning on unused term-level pattern names. Unlike
`-Wunused-matches`, `-Wunused-type-variables` is not implied by `-Wall`.

Fixes #11451.

Test Plan: ./validate

Reviewers: goldfire, ekmett, austin, hvr, simonpj, bgamari

Reviewed By: simonpj, bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1825

GHC Trac Issues: #11451

1 files changed, 12 insertions, 0 deletions

diff --git a/utils/mkUserGuidePart/Options/Warnings.hs b/utils/mkUserGuidePart/Options/Warnings.hs
index 256d01f9fa..3fa9bf087e 100644
--- a/utils/mkUserGuidePart/Options/Warnings.hs
+++ b/utils/mkUserGuidePart/Options/Warnings.hs
@@ -291,6 +291,18 @@ warningsOptions =
          , flagType = DynamicFlag
          , flagReverse = "-Wno-unused-matches"
          }
+  , flag { flagName = "-Wunused-foralls"
+         , flagDescription = "warn about type variables in user-written "++
+           "``forall``\\s that are unused"
+         , flagType = DynamicFlag
+         , flagReverse = "-Wno-unused-foralls"
+         }
+  , flag { flagName = "-Wunused-type-variables"
+         , flagDescription = "warn about variables in type family or data "++
+           "family instances that are unused"
+         , flagType = DynamicFlag
+         , flagReverse = "-Wno-unused-type-variables"
+         }
   , flag { flagName = "-Wunused-do-bind"
          , flagDescription =
            "warn about do bindings that appear to throw away values of types "++