Correctly report SrcLoc of redundant constraints

We were accidentally dropping the source location information in
certain circumstances when reporting redundant constraints. This patch
makes sure that we set the TcLclEnv correctly before reporting the
warning.

Fixes #21315

1 files changed, 12 insertions, 7 deletions

diff --git a/compiler/GHC/Tc/Errors.hs b/compiler/GHC/Tc/Errors.hs
index 9b9eb8077b..f662495f2c 100644
--- a/compiler/GHC/Tc/Errors.hs
+++ b/compiler/GHC/Tc/Errors.hs
@@ -393,17 +393,22 @@ warnRedundantConstraints ctxt env info ev_vars
  = return ()
 
  | SigSkol user_ctxt _ _ <- info
- = restoreLclEnv env $  -- We want to add "In the type signature for f"
-                        -- to the error context, which is a bit tiresome
+ -- When dealing with a user-written type signature,
+ -- we want to add "In the type signature for f".
+ = restoreLclEnv env $
    setSrcSpan (redundantConstraintsSpan user_ctxt) $
    report_redundant_msg True
+                  --  ^^^^ add "In the type signature..."
 
- | otherwise  -- But for InstSkol there already *is* a surrounding
-              -- "In the instance declaration for Eq [a]" context
-              -- and we don't want to say it twice. Seems a bit ad-hoc
- = report_redundant_msg False
+ | otherwise
+ -- But for InstSkol there already *is* a surrounding
+ -- "In the instance declaration for Eq [a]" context
+ -- and we don't want to say it twice. Seems a bit ad-hoc
+ = restoreLclEnv env
+ $ report_redundant_msg False
+                 --   ^^^^^ don't add "In the type signature..."
  where
-   report_redundant_msg :: Bool -- whether to add "In ..." to the diagnostic
+   report_redundant_msg :: Bool -- whether to add "In the type signature..." to the diagnostic
                         -> TcRn ()
    report_redundant_msg show_info
      = do { lcl_env <- getLclEnv