Make deferal failures into a kind of soft fail rather than an exception.

src/builtins/control.pir

@@ -444,6 +444,7 @@ on error.
     .local pmc clist, lexpad, self, next
     get_next_candidate_info clist, $P0, lexpad
     next = clone clist
+    next.'set_failure_mode'()
     $P0 = deref next
     $I0 = isa $P0, 'Method'
     unless $I0 goto not_method
@@ -467,14 +468,26 @@ on error.
     .local pmc clist, lexpad, self, next, result
     get_next_candidate_info clist, $P0, lexpad
     next = clone clist
+    next.'set_failure_mode'()
     $P0 = deref next
     $I0 = isa $P0, 'Method'
     unless $I0 goto not_method
     self = lexpad['self']
     (result) = next(self, pos_args :flat, named_args :flat :named)
-    'return'(result)
+    goto process_result
   not_method:
     (result) = next(pos_args :flat, named_args :flat :named)
+
+  process_result:
+    $I0 = isa result, ['Failure']
+    unless $I0 goto did_defer
+    $P0 = getattribute result, '$!exception'
+    if null $P0 goto did_defer
+    $S0 = $P0['message']
+    if $S0 != 'No method to defer to' goto did_defer
+    .return (result)
+
+  did_defer:
     'return'(result)
 .end
 
@@ -493,6 +506,7 @@ on error.
     # and tailcall the next candidate.
     .local pmc pos_args, named_args
     (pos_args, named_args) = '!get_original_args'(routine, lexpad)
+    next.'set_failure_mode'()
     .tailcall next(pos_args :flat, named_args :flat :named)
 .end
 
@@ -509,10 +523,21 @@ on error.
 
     # Build arguments based upon what the caller was originall invoked with,
     # get the result of the next candidate and use return to retrun from
-    # the caller.
+    # the caller, provided the defer did not fail.
     .local pmc pos_args, named_args, result
     (pos_args, named_args) = '!get_original_args'(routine, lexpad)
+    next.'set_failure_mode'()
     (result) = next(pos_args :flat, named_args :flat :named)
+
+    $I0 = isa result, ['Failure']
+    unless $I0 goto did_defer
+    $P0 = getattribute result, '$!exception'
+    if null $P0 goto did_defer
+    $S0 = $P0['message']
+    if $S0 != 'No method to defer to' goto did_defer
+    .return (result)
+
+  did_defer:
     'return'(result)
 .end
 

src/builtins/guts.pir

@@ -1535,6 +1535,21 @@ return the resume continuation so we can continue execution after the bind.
     .return ($P0)
 .end
 
+
+=item !deferal_fail
+
+Used by P6invocation to help us get soft-failure semantics when no deferal
+is possible.
+
+=cut
+
+.sub '!deferal_fail'
+    .param pmc pos_args    :slurpy
+    .param pmc named_args  :slurpy :named
+    .lex '__CANDIDATE_LIST__', $P0
+    .tailcall '!FAIL'('No method to defer to')
+.end
+
 =back
 
 =cut

src/pmc/p6invocation.pmc

@@ -22,9 +22,17 @@ gets stuck into the lex pad to represent the the candidate list.
 #include "pmc_perl6multisub.h"
 
 
+/* Selects whether we're doing a real dispatch or just a test to see if
+ * we could dispatch. */
 #define P6I_MODE_DISPATCH 0
 #define P6I_MODE_CHECK    1
 
+
+/* Flag on the PMC to say whether we're in quiet failure mode (if set)
+ * or exception mode if we are unable to dispatch. */
+#define PObj_P6I_FAILURE_MODE_FLAG PObj_private0_FLAG
+
+
 /* Declaration; definition of this is in p6opaque.pmc. */
 PMC *look_for_method(PARROT_INTERP, PMC *search_list, INTVAL *start_pos, STRING *name);
 
@@ -165,6 +173,8 @@ pmclass P6Invocation need_ext dynpmc group perl6_group {
         SETATTR_P6Invocation_search_list(interp, copy, search_list);
         SETATTR_P6Invocation_name(interp, copy, name);
         SETATTR_P6Invocation_resume_point(interp, copy, resume_point);
+        if (PObj_flag_TEST(P6I_FAILURE_MODE, SELF))
+            PObj_flag_SET(P6I_FAILURE_MODE, copy);
         return copy;
     }
 
@@ -220,8 +230,19 @@ pmclass P6Invocation need_ext dynpmc group perl6_group {
             SETATTR_P6Invocation_first_candidate(interp, SELF, PMCNULL);
 
         /* Invoke it, then fudge ourself into its lexpad. */
-        if (PMC_IS_NULL(first_candidate))
-            Parrot_ex_throw_from_c_args(interp, next, 1, "No candidates found to invoke");
+        if (PMC_IS_NULL(first_candidate)) {
+            PMC *ns;
+
+            /* Oh noes, no candidate. If we aren't in soft-fail mode, then
+             * throw an exception. */
+            if (!PObj_flag_TEST(P6I_FAILURE_MODE, SELF))
+                Parrot_ex_throw_from_c_args(interp, next, 1, "No candidates found to invoke");
+
+            /* Otherwise, we look up something that when invoked will just give a
+             * dispatch failure. */
+            ns = Parrot_get_namespace_keyed_str(interp, interp->root_namespace, CONST_STRING(interp, "perl6"));
+            first_candidate = VTABLE_get_pmc_keyed_str(interp, ns, CONST_STRING(interp, "!deferal_fail"));
+        }
         addr = VTABLE_invoke(interp, first_candidate, next);
         lexpad = CONTEXT(interp)->lex_pad;
         if (!PMC_IS_NULL(lexpad) && VTABLE_exists_keyed_str(interp, lexpad, lexname))
@@ -259,4 +280,8 @@ pmclass P6Invocation need_ext dynpmc group perl6_group {
         SETATTR_P6Invocation_search_list(interp, SELF, PMCNULL);
         SETATTR_P6Invocation_resume_point(interp, SELF, 0);
     }
+
+    METHOD void set_failure_mode() {
+        PObj_flag_SET(P6I_FAILURE_MODE, SELF);
+    }
 }