Don't think declare -x -F var is used (fixes #1209).

src/ShellCheck/Analytics.hs

@@ -1800,6 +1800,7 @@ prop_checkUnused35= verifyNotTree checkUnusedAssignments "a=foo; b=2; echo ${a:b
 prop_checkUnused36= verifyNotTree checkUnusedAssignments "if [[ -v foo ]]; then true; fi"
 prop_checkUnused37= verifyNotTree checkUnusedAssignments "fd=2; exec {fd}>&-"
 prop_checkUnused38= verifyTree checkUnusedAssignments "(( a=42 ))"
+prop_checkUnused39= verifyNotTree checkUnusedAssignments "declare -x -f foo"
 checkUnusedAssignments params t = execWriter (mapM_ warnFor unused)
   where
     flow = variableFlow params

src/ShellCheck/AnalyzerLib.hs

@@ -498,7 +498,9 @@ getReferencedVariableCommand base@(T_SimpleCommand _ _ (T_NormalWord _ (T_Litera
         "export" -> if "f" `elem` flags
             then []
             else concatMap getReference rest
-        "declare" -> if any (`elem` flags) ["x", "p"]
+        "declare" -> if
+                any (`elem` flags) ["x", "p"] &&
+                    (not $ any (`elem` flags) ["f", "F"])
             then concatMap getReference rest
             else []
         "readonly" ->