Iter 16c — literal patterns via desugar

Desugar `Pattern::Lit` (top-level and nested in Ctor) to
`Term::If { cond = (== sv lit), then = body, else_ = fall_k }`.
After 16c, no Pattern::Lit survives the desugar pass; codegen
and typechecker never see one. `is_flat` reclassifies Lit as
non-flat to force the chain machinery; new `build_eq` helper
constructs the equality test per literal kind.

Tests: 99 → 103 (+1 e2e lit_pat_demo, +3 desugar unit). Demo
fixture exercises top-level lit arms (classify) and nested
lit-in-Ctor (categorize_first) with a local IntList; output
100/200/999/-1/0/7.

Known limitations documented in DESIGN.md and JOURNAL: (a)
chain machinery's Unit terminator means non-Wild-terminated
exhaustive matches with lit arms still need a trailing `_`
catch-all to type-check; (b) `==` is currently Int-only, so
Bool/Str/Unit lit patterns desugar correctly but error at
typecheck.

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

examples/lit_pat.ail.json

@@ -0,0 +1 @@
+{"defs":[{"ctors":[{"fields":[],"name":"Nil"},{"fields":[{"k":"con","name":"Int"},{"k":"con","name":"IntList"}],"name":"Cons"}],"kind":"type","name":"IntList"},{"body":{"arms":[{"body":{"lit":{"kind":"int","value":100},"t":"lit"},"pat":{"lit":{"kind":"int","value":0},"p":"lit"}},{"body":{"lit":{"kind":"int","value":200},"t":"lit"},"pat":{"lit":{"kind":"int","value":1},"p":"lit"}},{"body":{"lit":{"kind":"int","value":999},"t":"lit"},"pat":{"p":"wild"}}],"scrutinee":{"name":"n","t":"var"},"t":"match"},"doc":"Map 0->100, 1->200, anything else->999.","kind":"fn","name":"classify","params":["n"],"type":{"effects":[],"k":"fn","params":[{"k":"con","name":"Int"}],"ret":{"k":"con","name":"Int"}}},{"body":{"arms":[{"body":{"lit":{"kind":"int","value":-1},"t":"lit"},"pat":{"ctor":"Nil","fields":[],"p":"ctor"}},{"body":{"lit":{"kind":"int","value":0},"t":"lit"},"pat":{"ctor":"Cons","fields":[{"lit":{"kind":"int","value":0},"p":"lit"},{"p":"wild"}],"p":"ctor"}},{"body":{"name":"h","t":"var"},"pat":{"ctor":"Cons","fields":[{"name":"h","p":"var"},{"p":"wild"}],"p":"ctor"}},{"body":{"lit":{"kind":"int","value":0},"t":"lit"},"pat":{"p":"wild"}}],"scrutinee":{"name":"xs","t":"var"},"t":"match"},"doc":"Empty -> -1; first element 0 -> 0; otherwise return the head.","kind":"fn","name":"categorize_first","params":["xs"],"type":{"effects":[],"k":"fn","params":[{"k":"con","name":"IntList"}],"ret":{"k":"con","name":"Int"}}},{"body":{"lhs":{"args":[{"args":[{"lit":{"kind":"int","value":0},"t":"lit"}],"fn":{"name":"classify","t":"var"},"t":"app"}],"op":"io/print_int","t":"do"},"rhs":{"lhs":{"args":[{"args":[{"lit":{"kind":"int","value":1},"t":"lit"}],"fn":{"name":"classify","t":"var"},"t":"app"}],"op":"io/print_int","t":"do"},"rhs":{"lhs":{"args":[{"args":[{"lit":{"kind":"int","value":17},"t":"lit"}],"fn":{"name":"classify","t":"var"},"t":"app"}],"op":"io/print_int","t":"do"},"rhs":{"lhs":{"args":[{"args":[{"args":[],"ctor":"Nil","t":"ctor","type":"IntList"}],"fn":{"name":"categorize_first","t":"var"},"t":"app"}],"op":"io/print_int","t":"do"},"rhs":{"lhs":{"args":[{"args":[{"args":[{"lit":{"kind":"int","value":0},"t":"lit"},{"args":[],"ctor":"Nil","t":"ctor","type":"IntList"}],"ctor":"Cons","t":"ctor","type":"IntList"}],"fn":{"name":"categorize_first","t":"var"},"t":"app"}],"op":"io/print_int","t":"do"},"rhs":{"args":[{"args":[{"args":[{"lit":{"kind":"int","value":7},"t":"lit"},{"args":[],"ctor":"Nil","t":"ctor","type":"IntList"}],"ctor":"Cons","t":"ctor","type":"IntList"}],"fn":{"name":"categorize_first","t":"var"},"t":"app"}],"op":"io/print_int","t":"do"},"t":"seq"},"t":"seq"},"t":"seq"},"t":"seq"},"t":"seq"},"doc":"Drive both fns. Expected (per line): 100, 200, 999, -1, 0, 7.","kind":"fn","name":"main","params":[],"type":{"effects":["IO"],"k":"fn","params":[],"ret":{"k":"con","name":"Unit"}}}],"imports":[],"name":"lit_pat","schema":"ailang/v0"}

examples/lit_pat.ailx

@@ -0,0 +1,52 @@
+; Iter 16c — lit patterns at top level and inside Ctor sub-patterns.
+; `classify` matches an Int against three lit arms (0, 1, default).
+; `categorize_first` matches a List<Int>: empty -> -1, head==0 -> 0,
+; otherwise the head value. The latter exercises Pat-Lit nested
+; inside Pat-Ctor's first field. Both shapes are now handled by the
+; 16c desugar pass; codegen sees only If/Match-on-Ctor combinations.
+;
+; The trailing `_` catch-all in `categorize_first` is required by the
+; 16a chain machinery: the chain's terminator is a `Unit` literal,
+; reachable when no arm matches. A `_` arm dominates the chain so the
+; terminator stays unreachable in practice and the match type-checks.
+;
+; Expected stdout (one per line): 100, 200, 999, -1, 0, 7.
+
+(module lit_pat
+
+  (data IntList
+    (ctor Nil)
+    (ctor Cons (con Int) (con IntList)))
+
+  (fn classify
+    (doc "Map 0->100, 1->200, anything else->999.")
+    (type (fn-type (params (con Int)) (ret (con Int))))
+    (params n)
+    (body
+      (match n
+        (case (pat-lit 0) 100)
+        (case (pat-lit 1) 200)
+        (case _ 999))))
+
+  (fn categorize_first
+    (doc "Empty -> -1; first element 0 -> 0; otherwise return the head.")
+    (type (fn-type (params (con IntList)) (ret (con Int))))
+    (params xs)
+    (body
+      (match xs
+        (case (pat-ctor Nil) -1)
+        (case (pat-ctor Cons (pat-lit 0) _) 0)
+        (case (pat-ctor Cons h _) h)
+        (case _ 0))))
+
+  (fn main
+    (doc "Drive both fns. Expected (per line): 100, 200, 999, -1, 0, 7.")
+    (type (fn-type (params) (ret (con Unit)) (effects IO)))
+    (params)
+    (body
+      (seq (do io/print_int (app classify 0))
+      (seq (do io/print_int (app classify 1))
+      (seq (do io/print_int (app classify 17))
+      (seq (do io/print_int (app categorize_first (term-ctor IntList Nil)))
+      (seq (do io/print_int (app categorize_first (term-ctor IntList Cons 0 (term-ctor IntList Nil))))
+           (do io/print_int (app categorize_first (term-ctor IntList Cons 7 (term-ctor IntList Nil))))))))))))