Document -fwarn-missing-import-lists

Also change the behaviour slightly, to warn only for
*unqualified* imports.  See Trac #4977.

compiler/rename/RnNames.lhs

@@ -112,8 +112,9 @@ rnImportDecl this_mod implicit_prelude
 	-- (Opt_WarnMissingImportList also checks for T(..) items
 	--  but that is done in checkDodgyImport below)
     case imp_details of
-        Just (False, _)       -> return ()
+        Just (False, _)       -> return ()	-- Explicit import list
         _  | implicit_prelude -> return ()
+           | qual_only	      -> return ()
            | otherwise        -> ifDOptM Opt_WarnMissingImportList $
                                  addWarn (missingImportListWarn imp_mod_name)
 
@@ -586,6 +587,7 @@ filterImports iface decl_spec (Just (want_hiding, import_items)) all_avails
                 = ifDOptM Opt_WarnDodgyImports (addWarn (dodgyImportWarn n))
                 -- NB. use the RdrName for reporting the warning
 		| IEThingAll {} <- ieRdr
+		, not (is_qual decl_spec)
                 = ifDOptM Opt_WarnMissingImportList $
                   addWarn (missingImportListItem ieRdr)
             checkDodgyImport _

docs/users_guide/using.xml

@@ -1299,6 +1299,36 @@ f foo = foo { x = 6 }
 	</listitem>
       </varlistentry>
 
+      <varlistentry>
+	<term>
+          <option>-fwarn-missing-import-lists</option>:
+	  <indexterm><primary><option>-fwarn-import-lists</option></primary></indexterm>
+	  <indexterm><primary>missing import lists, warning</primary></indexterm>
+	  <indexterm><primary>import lists, missing</primary></indexterm>
+        </term>
+	<listitem>
+
+	  <para>This flag warns if you use an unqualified 
+            <literal>import</literal> declaration
+	    that does not explicitly list the entities brought into scope. For 
+	    example
+<programlisting>
+module M where
+  import X( f )
+  import Y
+  import qualified Z
+  p x = f x x
+<programlisting>
+          The <option>-fwarn-import-lists</option> flag will warn about the import
+	  of <literal>Y</literal> but not <literal>X</literal>
+	  If module <literal>Y</literal> is later changed to export (say) <literal>f</literal>,
+          then the reference to <literal>f</literal> in <literal>M</literal> will become
+	  ambiguous.  No warning is produced for the import of <literal>Z</literal>
+	  because extending <literal>Z</literal>'s exports would be unlikely to produce
+	  ambiguity in <literal>M</literal>.
+	</listitem>
+      </varlistentry>
+
       <varlistentry>
 	<term><option>-fwarn-missing-methods</option>:</term>
 	<listitem>