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`instance forall c. c` claimed that `c` was out of scope because the renamer was invoking `lookupGlobalOcc` on `c` (in `RnNames.getLocalNonValBinders`) without binding `c` first. To avoid this, this patch changes GHC to invoke `lookupGlobalOcc_maybe` on `c` instead, and if that returns `Nothing`, then bail out, resulting in a better error message.
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