Instance declaration overlap allowed if *either* has -XOverlappingInstances

This satisfies Trac #3877.  Documentation is changed too.
I'm not sure if this should go in 7.0.2.

diff --git a/compiler/types/InstEnv.lhs b/compiler/types/InstEnv.lhs
index dfbe0a0..34bd5eb 100644 (file)
--- a/compiler/types/InstEnv.lhs
+++ b/compiler/types/InstEnv.lhs
@@ -533,14 +533,18 @@ insert_overlapping new_item (item:items)
     old_beats_new = item `beats` new_item
 
     (instA, _) `beats` (instB, _)
-       = overlap_ok && 
-         isJust (tcMatchTys (is_tvs instB) (is_tys instB) (is_tys instA))
-               -- A beats B if A is more specific than B, and B admits overlap
-               -- I.e. if B can be instantiated to match A
-       where
-         overlap_ok = case is_flag instB of
-                       NoOverlap -> False
-                       _         -> True
+          = overlap_ok && 
+            isJust (tcMatchTys (is_tvs instB) (is_tys instB) (is_tys instA))
+                    -- A beats B if A is more specific than B,
+                    -- (ie. if B can be instantiated to match A)
+                    -- and overlap is permitted
+          where
+            -- Overlap permitted if *either* instance permits overlap
+            -- This is a change (Trac #3877, Dec 10). It used to
+            -- require that instB (the less specific one) permitted overlap.
+            overlap_ok = case (is_flag instA, is_flag instB) of
+                              (NoOverlap, NoOverlap) -> False
+                              _                      -> True
 \end{code}
 
 

diff --git a/docs/users_guide/glasgow_exts.xml b/docs/users_guide/glasgow_exts.xml
index e57b294..a193088 100644 (file)
--- a/docs/users_guide/glasgow_exts.xml
+++ b/docs/users_guide/glasgow_exts.xml
@@ -4004,18 +4004,21 @@ The willingness to be overlapped or incoherent is a property of
 the <emphasis>instance declaration</emphasis> itself, controlled by the
 presence or otherwise of the <option>-XOverlappingInstances</option> 
 and <option>-XIncoherentInstances</option> flags when that module is
-being defined.  Neither flag is required in a module that imports and uses the
-instance declaration.  Specifically, during the lookup process:
+being defined.  Specifically, during the lookup process:
 <itemizedlist>
 <listitem><para>
-An instance declaration is ignored during the lookup process if (a) a more specific
-match is found, and (b) the instance declaration was compiled with 
-<option>-XOverlappingInstances</option>.  The flag setting for the
-more-specific instance does not matter.
+If the constraint being looked up matches two instance declarations IA and IB,
+and
+<itemizedlist>
+<listitem><para>IB is a substitution instance of IA (but not vice versa);
+that is, IB is strictly more specific than IA</para></listitem>
+<listitem><para>either IA or IB was compiled with <option>-XOverlappingInstances</option></para></listitem>
+</itemizedlist>
+then the less-specific instance IA is ignored.
 </para></listitem>
 <listitem><para>
 Suppose an instance declaration does not match the constraint being looked up, but
-does unify with it, so that it might match when the constraint is further 
+does <emphasis>unify</emphasis> with it, so that it might match when the constraint is further
 instantiated.  Usually GHC will regard this as a reason for not committing to
 some other constraint.  But if the instance declaration was compiled with
 <option>-XIncoherentInstances</option>, GHC will skip the "does-it-unify?" 
@@ -4025,18 +4028,6 @@ check for that declaration.
 These rules make it possible for a library author to design a library that relies on 
 overlapping instances without the library client having to know.  
 </para>
-<para>
-If an instance declaration is compiled without
-<option>-XOverlappingInstances</option>,
-then that instance can never be overlapped.  This could perhaps be
-inconvenient.  Perhaps the rule should instead say that the
-<emphasis>overlapping</emphasis> instance declaration should be compiled in
-this way, rather than the <emphasis>overlapped</emphasis> one.  Perhaps overlap
-at a usage site should be permitted regardless of how the instance declarations
-are compiled, if the <option>-XOverlappingInstances</option> flag is
-used at the usage site.  (Mind you, the exact usage site can occasionally be
-hard to pin down.)  We are interested to receive feedback on these points.
-</para>
 <para>The <option>-XIncoherentInstances</option> flag implies the
 <option>-XOverlappingInstances</option> flag, but not vice versa.
 </para>