Iter 16b.4: LetRec captures of match-arm pattern bindings

Flips the desugar's MatchArm-capture rejection from panic to defer.
The 16b.3 lift_letrecs pass already handled match-arm bindings via
type_check_pattern_for_lift (Pattern::Var, Pattern::Ctor with sub-
patterns, ctor-field substitution against scrutinee args), so this
iter is a single-line classification change in desugar plus the
fixture and tests that exercise it.

- desugar.rs: MatchArm classification now defers (same arm as
  LetBound); EnclosingLetRec panic remains for 16b.7.
- examples/local_rec_match_capture.{ailx,ail.json}: enclosing fn
  pattern-matches on Pair<Int,Int>, inner LetRec captures both
  match-arm bindings simultaneously. Lifts to
  loop$lr_0(i: Int, threshold: Int, n: Int) -> Int.
- e2e + check + desugar tests: 110 → 113 (+3).

First fixture lifting more than one capture; subst_call_with_extras
already handled it generically.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

examples/local_rec_match_capture.ail.json

@@ -0,0 +1 @@
+{"defs":[{"ctors":[{"fields":[{"k":"var","name":"a"},{"k":"var","name":"b"}],"name":"MkPair"}],"kind":"type","name":"Pair","vars":["a","b"]},{"body":{"arms":[{"body":{"body":{"cond":{"args":[{"name":"i","t":"var"},{"name":"n","t":"var"}],"fn":{"name":">","t":"var"},"t":"app"},"else":{"cond":{"args":[{"name":"i","t":"var"},{"name":"threshold","t":"var"}],"fn":{"name":"<","t":"var"},"t":"app"},"else":{"args":[{"args":[{"name":"i","t":"var"},{"lit":{"kind":"int","value":1},"t":"lit"}],"fn":{"name":"+","t":"var"},"t":"app"}],"fn":{"name":"loop","t":"var"},"t":"app"},"t":"if","then":{"args":[{"lit":{"kind":"int","value":1},"t":"lit"},{"args":[{"args":[{"name":"i","t":"var"},{"lit":{"kind":"int","value":1},"t":"lit"}],"fn":{"name":"+","t":"var"},"t":"app"}],"fn":{"name":"loop","t":"var"},"t":"app"}],"fn":{"name":"+","t":"var"},"t":"app"}},"t":"if","then":{"lit":{"kind":"int","value":0},"t":"lit"}},"in":{"args":[{"lit":{"kind":"int","value":1},"t":"lit"}],"fn":{"name":"loop","t":"var"},"t":"app"},"name":"loop","params":["i"],"t":"letrec","type":{"effects":[],"k":"fn","params":[{"k":"con","name":"Int"}],"ret":{"k":"con","name":"Int"}}},"pat":{"ctor":"MkPair","fields":[{"name":"threshold","p":"var"},{"name":"n","p":"var"}],"p":"ctor"}}],"scrutinee":{"name":"p","t":"var"},"t":"match"},"doc":"Match-arm captures `threshold` and `n`; inner LetRec captures both.","kind":"fn","name":"count_below","params":["p"],"type":{"effects":[],"k":"fn","params":[{"args":[{"k":"con","name":"Int"},{"k":"con","name":"Int"}],"k":"con","name":"Pair"}],"ret":{"k":"con","name":"Int"}}},{"body":{"lhs":{"args":[{"args":[{"args":[{"lit":{"kind":"int","value":10},"t":"lit"},{"lit":{"kind":"int","value":0},"t":"lit"}],"ctor":"MkPair","t":"ctor","type":"Pair"}],"fn":{"name":"count_below","t":"var"},"t":"app"}],"op":"io/print_int","t":"do"},"rhs":{"lhs":{"args":[{"args":[{"args":[{"lit":{"kind":"int","value":10},"t":"lit"},{"lit":{"kind":"int","value":5},"t":"lit"}],"ctor":"MkPair","t":"ctor","type":"Pair"}],"fn":{"name":"count_below","t":"var"},"t":"app"}],"op":"io/print_int","t":"do"},"rhs":{"args":[{"args":[{"args":[{"lit":{"kind":"int","value":10},"t":"lit"},{"lit":{"kind":"int","value":15},"t":"lit"}],"ctor":"MkPair","t":"ctor","type":"Pair"}],"fn":{"name":"count_below","t":"var"},"t":"app"}],"op":"io/print_int","t":"do"},"t":"seq"},"t":"seq"},"doc":"Drive count_below at MkPair 10 {0,5,15}. Expected: 0, 5, 9.","kind":"fn","name":"main","params":[],"type":{"effects":["IO"],"k":"fn","params":[],"ret":{"k":"con","name":"Unit"}}}],"imports":[],"name":"local_rec_match_capture","schema":"ailang/v0"}

examples/local_rec_match_capture.ailx

@@ -0,0 +1,59 @@
+; Iter 16b.4 — LetRec captures of match-arm pattern bindings.
+; `count_below(p)` takes a `Pair Int Int` (threshold, n), pattern-
+; matches on `(MkPair threshold n)`, and inside that match arm runs
+; a recursive helper `loop` that captures BOTH match-arm bindings
+; (`threshold` and `n`) and counts how many integers in 1..=n are
+; strictly less than `threshold`.
+;
+; The 16b.2/16b.3 desugar passes cannot lift this LetRec — both
+; `threshold` and `n` are bound by a `Pattern::Ctor` Var sub-
+; pattern (`ScopeEntry::MatchArm`). Before 16b.4 this panicked at
+; desugar time. Now the desugar pass leaves the LetRec in place;
+; the post-typecheck `lift_letrecs` pass walks the enclosing match,
+; resolves each pattern binding's type via constructor-field
+; substitution against the matched ctor's declared field types
+; (here: `Pair Int Int`'s ctor `MkPair` has fields `[Int, Int]`,
+; and the scrutinee's `Type::Con.args` are also `[Int, Int]`, so
+; substitution is a no-op and both bindings get type `Int`), and
+; lifts to `loop$lr_0(i: Int, threshold: Int, n: Int) -> Int`.
+;
+; The match has a single ctor and no catch-all; the chain machinery
+; treats the only arm as default-dominating since there's no `_`
+; arm needed for an exhaustive single-ctor ADT (Pair has one ctor).
+;
+; Expected stdout (one per line):
+;   count_below(MkPair 10 0)  = 0
+;   count_below(MkPair 10 5)  = 5  (1..=5 all below 10)
+;   count_below(MkPair 10 15) = 9  (1..=9 below 10, 10..=15 not)
+
+(module local_rec_match_capture
+
+  (data Pair (vars a b)
+    (ctor MkPair a b))
+
+  (fn count_below
+    (doc "Match-arm captures `threshold` and `n`; inner LetRec captures both.")
+    (type (fn-type (params (con Pair (con Int) (con Int))) (ret (con Int))))
+    (params p)
+    (body
+      (match p
+        (case (pat-ctor MkPair threshold n)
+          (let-rec loop
+            (params i)
+            (type (fn-type (params (con Int)) (ret (con Int))))
+            (body
+              (if (app > i n)
+                0
+                (if (app < i threshold)
+                  (app + 1 (app loop (app + i 1)))
+                  (app loop (app + i 1)))))
+            (in (app loop 1)))))))
+
+  (fn main
+    (doc "Drive count_below at MkPair 10 {0,5,15}. Expected: 0, 5, 9.")
+    (type (fn-type (params) (ret (con Unit)) (effects IO)))
+    (params)
+    (body
+      (seq (do io/print_int (app count_below (term-ctor Pair MkPair 10 0)))
+      (seq (do io/print_int (app count_below (term-ctor Pair MkPair 10 5)))
+           (do io/print_int (app count_below (term-ctor Pair MkPair 10 15))))))))