[m-rev.] diff: fix the failure of valid/unify_typeinfo_bug in the java grade

[Julien Fischer]

Fix the failure of valid/unify_typeinfo_bug in the java grade.

The above test is failing because the runtime does not define a specialised
method pointer interface for arity zero predicates but the code generator
thinks that that it does.  The fix is to change the code generator to use the
generic method pointer interface for zero arity predicates.  (Zero arity
predicates are rare enough that there is no point in having a specialised
interface for them.)

compiler/mlds_to_java.m:
      Make the above change.

Julien.

diff --git a/compiler/mlds_to_java.m b/compiler/mlds_to_java.m
index a8300b1..fb780db 100644
--- a/compiler/mlds_to_java.m
+++ b/compiler/mlds_to_java.m
@@ -851,7 +851,7 @@ generate_addr_wrapper_class(MLDS_ModuleName, Arity - CodeAddrs, ClassDefn,
      % Create a method that calls the original predicates.
      generate_call_method(Arity, CodeAddrs, MethodDefn),

-    ( if Arity =< max_specialised_method_ptr_arity then
+    ( if is_specialised_method_ptr_arity(Arity) then
          InterfaceName = "MethodPtr" ++ string.from_int(Arity)
      else
          InterfaceName = "MethodPtrN"
@@ -877,6 +877,12 @@ generate_addr_wrapper_class(MLDS_ModuleName, Arity - CodeAddrs, ClassDefn,

      add_to_address_map(ClassName, CodeAddrs, !AddrOfMap).

+:- pred is_specialised_method_ptr_arity(int::in) is semidet.
+
+is_specialised_method_ptr_arity(Arity) :-
+    Arity > 0,  % No specialised method ptr for arity zero predicates.
+    Arity =< max_specialised_method_ptr_arity.
+
      % The highest arity for which there is a specialised MethodPtr<n>
      % interface.
      %
@@ -891,7 +897,7 @@ generate_call_method(Arity, CodeAddrs, MethodDefn) :-
      % Create the arguments to the call method. For low arities, the method
      % takes n arguments directly. For higher arities, the arguments are
      % passed in as an array.
-    ( if Arity =< max_specialised_method_ptr_arity then
+    ( if is_specialised_method_ptr_arity(Arity) then
          list.map2(create_generic_arg, 1 .. Arity, ArgNames, MethodArgs),
          InputArgs = cmi_separate(ArgNames)
      else
@@ -3322,7 +3328,7 @@ output_stmt_call_for_java(Info, Indent, _FuncInfo, Stmt,
          ),

          list.length(CallArgs, Arity),
-        ( if Arity =< max_specialised_method_ptr_arity then
+        ( if is_specialised_method_ptr_arity(Arity) then
              io.write_string("((jmercury.runtime.MethodPtr", !IO),
              io.write_int(Arity, !IO),
              io.write_string(") ", !IO),