Fix Trac #10618 (out of scope operator)

Out of scope variables now generate HsUnboundVar,
and the fixity re-jigging wasn't taking this into
account.

compiler/rename/RnTypes.hs

@@ -829,8 +829,11 @@ mkOpAppRn e1 op fix e2                  -- Default case, no rearrangment
 
 ----------------------------
 get_op :: LHsExpr Name -> Name
-get_op (L _ (HsVar n)) = n
-get_op other           = pprPanic "get_op" (ppr other)
+-- An unbound name could be either HsVar or HsUnboundVra
+-- See RnExpr.rnUnboundVar
+get_op (L _ (HsVar n))          = n
+get_op (L _ (HsUnboundVar occ)) = mkUnboundName (mkRdrUnqual occ)
+get_op other                    = pprPanic "get_op" (ppr other)
 
 -- Parser left-associates everything, but
 -- derived instances may have correctly-associated things to

testsuite/tests/rename/should_fail/T10618.hs

+module T10618 where
+
+foo = Just $ Nothing <> Nothing

testsuite/tests/rename/should_fail/T10618.stderr

+
+T10618.hs:3:22: error:
+    Variable not in scope: (<>) :: Maybe (Maybe a0) -> Maybe a1 -> t
+    Perhaps you meant one of these:
+      ‘<$>’ (imported from Prelude), ‘*>’ (imported from Prelude),
+      ‘<$’ (imported from Prelude)

testsuite/tests/rename/should_fail/all.T

@@ -133,3 +133,4 @@ test('T9032',
      normal,
      run_command,
      ['$MAKE -s --no-print-directory T9032'])
+test('T10618', normal, compile_fail, [''])