Add a -Warn flag

compiler/main/DynFlags.hs

@@ -1088,12 +1088,13 @@ dynamic_flags = [
   ,  ( "monly-3-regs", 	NoArg (upd (\s -> s{stolen_x86_regs = 3}) ))
   ,  ( "monly-4-regs", 	NoArg (upd (\s -> s{stolen_x86_regs = 4}) ))
 
-	------ Warning opts -------------------------------------------------
-  ,  ( "W"		, NoArg (mapM_ setDynFlag   minusWOpts)    )
-  ,  ( "Werror"		, NoArg (setDynFlag   	    Opt_WarnIsError) )
-  ,  ( "Wall"		, NoArg (mapM_ setDynFlag   minusWallOpts) )
-  ,  ( "Wnot"		, NoArg (mapM_ unSetDynFlag minusWallOpts) ) /* DEPREC */
-  ,  ( "w"		, NoArg (mapM_ unSetDynFlag minuswRemovesOpts) )
+     ------ Warning opts -------------------------------------------------
+  ,  ( "W"     , NoArg (mapM_ setDynFlag   minusWOpts)    )
+  ,  ( "Werror", NoArg (setDynFlag         Opt_WarnIsError) )
+  ,  ( "Wwarn" , NoArg (unSetDynFlag       Opt_WarnIsError) )
+  ,  ( "Wall"  , NoArg (mapM_ setDynFlag   minusWallOpts) )
+  ,  ( "Wnot"  , NoArg (mapM_ unSetDynFlag minusWallOpts) ) -- DEPRECATED
+  ,  ( "w"     , NoArg (mapM_ unSetDynFlag minuswRemovesOpts) )
 
 	------ Optimisation flags ------------------------------------------
   ,  ( "O"	, NoArg (upd (setOptLevel 1)))

docs/users_guide/flags.xml

@@ -968,7 +968,13 @@
 	    <entry><option>-Werror</option></entry>
 	    <entry>make warnings fatal</entry>
 	    <entry>dynamic</entry>
-	    <entry></entry>
+	    <entry>-Wwarn</entry>
+	  </row>
+	  <row>
+	    <entry><option>-Wwarn</option></entry>
+	    <entry>make warnings non-fatal</entry>
+	    <entry>dynamic</entry>
+	    <entry>-Werror</entry>
 	  </row>
 
 	  <row>

docs/users_guide/using.xml

@@ -897,6 +897,16 @@ ghc -c Foo.hs</screen>
 	</listitem>
       </varlistentry>
 
+      <varlistentry>
+	<term><option>-Wwarn</option>:</term>
+	<listitem>
+	  <indexterm><primary><option>-Wwarn</option></primary></indexterm>
+	  <para>Warnings are treated only as warnings, not as errors. This is
+	    the default, but can be useful to negate a
+        <option>-Werror</option> flag.</para>
+	</listitem>
+      </varlistentry>
+
     </variablelist>
 
     <para>The full set of warning options is described below.  To turn