Attempt to put the try-delegate label after delegate{l} (failing tests)

interpreter/exec/eval.ml

@@ -236,7 +236,7 @@ let rec step (c : config) : config =
         let n1 = Lib.List32.length ts1 in
         let n2 = Lib.List32.length ts2 in
         let args, vs' = take n1 vs e.at, drop n1 vs e.at in
-        vs', [Label (n2, [], ([], [Delegate (x.it, (args, List.map plain es')) @@ e.at])) @@ e.at]
+        vs', [Delegate (x.it, (args, [Label (n2, [], ([], List.map plain es')) @@ e.at])) @@ e.at]
 
       | Drop, v :: vs' ->
         vs', []
@@ -639,7 +639,10 @@ let rec step (c : config) : config =
     | Rethrowing _, _ ->
       Crash.error e.at "undefined catch label"
 
-    | Delegating _, _ ->
+    | Delegating (0l, a, vs0), vs ->
+      vs, [Throwing (a, vs0) @@ e.at]
+
+    | Delegating (k,_, _),  _ ->
       Crash.error e.at "undefined delegate label"
 
     | Label (n, es0, (vs', [])), vs ->

proposals/exception-handling/Exceptions-formal-overview.md

@@ -180,21 +180,22 @@ catch T[val^n (throw a)] end ↪  val^n (throw a)
 
 
 F; val^n (try bt instr* delegate l)
-  ↪ F; label_m{} (delegate{l} val^n instr* end) end
+  ↪ F; delegate{l} (label_m{} val^n instr* end) end
   (if expand_F(bt) = [t1^n]→[t2^m])
 
 delegate{l} val* end ↪ val*
 
-label_m{} B^l[ delegate{l} T[val^n (throw a)] end ] end
+B^l[ delegate{l} T[val^n (throw a)] end ]
   ↪ val^n (throw a)
 ```
 
 Note that the last reduction step above is similar to the reduction of `br l` [1], if we look at the entire `delegate{l}...end` as the `br l`, but also doing a throw after it breaks.
 
-There is a subtle difference though. The instruction `br l` searches for the `l+1`th surrounding block and breaks out after that block. Because `delegate{l}` is always wrapped in its own `label_n{} ... end` [2], with the same lookup as for `br l` we end up breaking inside the `l+1`th surrounding block, and throwing there. So if that `l+1`th surrounding block is a try, we end up throwing in its "try code", and thus correctly getting delegated to that try's catches.
+There is a subtle difference though. The instruction `br l` searches for the `l+1` surrounding block and breaks out after that block.
+On the contrary, `delegate{l}` should "break and throw" _inside_ the `l+1` label, so we should break one label earlier.
+This is why the reduction step for `delegate{l}` has one label less.
 
 - [1] [The execution step for `br l`](https://webassembly.github.io/spec/core/exec/instructions.html#xref-syntax-instructions-syntax-instr-control-mathsf-br-l)
-- [2] The label that always wraps `delegate{l}...end` can be thought of as "level -1" and cannot be referred to by the delegate's label index `l`.
 
 ### Typing Rules for Administrative Instructions
 
@@ -211,8 +212,8 @@ S;C, labels [t2*] ⊢ instr1* : []→[t2*]
 S;C, labels [t2*] ⊢ catch{a? instr2*}* instr1* end : []→[t2*]
 
 S;C ⊢ instr* : []→[t*]
-C.labels[l+1] = [t0*]
-------------------------------------------------------
+C.labels[l] = [t0*]
+---------------------------------------
 S;C ⊢ delegate{l} instr* end : []→[t*]
 
 S ⊢ tag a : tag [t0*]→[]

test/core/try_delegate_minimal_fail.wast

@@ -0,0 +1,33 @@
+;; This test fails with the first approach to changing the position of the
+;; delegate label. We could ommit the outermost try-catch_all, and get the
+;; error:
+;;
+;;   uncaught exception: uncaught exception with args ([])
+;;
+;; As it is now, the error is:
+;;
+;;   Result: 27 : i32
+;;   Expect: 2 : i32
+;;   ../test/core/try_delegate_minimal_fail.wast:22.1-22.77: assertion failure: wrong return values
+;;
+;; These errors indicate that the delegated exception escaped its target and was
+;; caught by the one surrounding it.
+
+
+(module
+  (tag $tag)
+  (func (export "delegate-throw-direct") (param i32) (result i32)
+    (try $outermost (result i32)
+      (do
+        (try $innermost (result i32)
+          (do
+            (try (result i32)
+              (do
+                (local.get 0)
+                (if (then (throw $tag)) (else))
+                (i32.const 1))
+              (delegate $innermost)))  ;; delegate 0
+          (catch $tag (i32.const 2)))) ;; end innermost
+      (catch $tag (i32.const 27)))))   ;; end outermost
+
+(assert_return (invoke "delegate-throw-direct" (i32.const 1)) (i32.const 2))