feat(parser): trait method default bodies (left-factored) (#135 slice 5)

#135 slice 5. `trait_item` had `fn_sig SEMICOLON` (TraitFn) and a whole
`fn_decl` (TraitFnDefault) as separate productions; they share the long
prefix `visibility? FN name params return_type?`, the LR conflict
resolved toward the signature form, so a trait method with a default
body — `pub fn ne(ref self, ref other: Self) -> Bool { ... }` in
stdlib/traits.affine — failed at the `{`.

Fix by left-factoring: parse `fn_sig` once, then branch on SEMICOLON
(→ TraitFn) vs `fn_body` (→ TraitFnDefault built from the sig + body).
The two trait-method forms now differ purely on the next token — the
shared-prefix ambiguity is removed, not papered over. (`ref self`,
sig-only methods, and associated types already worked and are verified
non-regressed.) Trait defaults don't use `total`/`where` (none in
stdlib); those default to false/[].

Effect: traits.affine advances 12 → 124 (cleared 100+ lines; next
blocker is `while let` / `Vec::new()`, a distinct slice-4-class
construct). Two regression tests; full suite green (228).

Advances #135. Refs #128, #135.

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

Author: hyperpolymath

lib/parser.mly

@@ -552,7 +552,25 @@ supertraits:
 
 trait_item:
   | sig_ = fn_sig SEMICOLON { TraitFn sig_ }
-  | f = fn_decl { TraitFnDefault f }
+  /* Trait method *default body* (issue #135 slice 5).  Previously the two
+     forms were `fn_sig SEMICOLON` vs a whole `fn_decl`, which share the
+     long prefix `visibility? FN name params return_type?`; the LR conflict
+     resolved toward the signature form, so `pub fn ne(ref self, ...) ->
+     Bool { ... }` (stdlib/traits.affine) failed at the `{`.  Left-factor:
+     parse `fn_sig` once, then branch on SEMICOLON vs fn_body — no shared-
+     prefix ambiguity (the two now differ purely on the next token).
+     Trait defaults don't use `total`/`where` (none in stdlib); fd_total
+     = false, fd_where = []. */
+  | sig_ = fn_sig body = fn_body
+    { TraitFnDefault { fd_vis = sig_.fs_vis;
+                       fd_total = false;
+                       fd_name = sig_.fs_name;
+                       fd_type_params = sig_.fs_type_params;
+                       fd_params = sig_.fs_params;
+                       fd_ret_ty = sig_.fs_ret_ty;
+                       fd_eff = sig_.fs_eff;
+                       fd_where = [];
+                       fd_body = body } }
   | TYPE name = ident kind = kind_annotation? default = type_default? SEMICOLON
     { TraitType { tt_name = name; tt_kind = kind; tt_default = default } }
 

test/test_e2e.ml

@@ -3342,6 +3342,27 @@ let test_bare_effect_and_effect_row () =
          fn q() -> Int / io { return 0; }
          fn plain() -> Int { return 1; }|})
 
+(* Issue #135 slice 5: trait method *default body* (left-factored vs the
+   signature form so the shared prefix no longer mis-resolves toward the
+   `;` form). `ref self` receiver + sig-only + assoc type unaffected. *)
+let test_trait_default_body () =
+  Alcotest.(check bool) "trait fn with default body + ref self" true
+    (parse_check_passes
+       {|trait Eq {
+           pub fn eq(ref self, ref other: Self) -> Bool;
+           pub fn ne(ref self, ref other: Self) -> Bool {
+             return !self.eq(other);
+           }
+         }|})
+
+let test_trait_sig_and_assoc_not_regressed () =
+  Alcotest.(check bool) "sig-only trait fn + associated type still parse" true
+    (parse_check_passes
+       {|trait Iter {
+           type Item;
+           pub fn next(mut self) -> Option<Int>;
+         }|})
+
 let test_multi_arg_arrow () =
   Alcotest.(check bool) "(A, B) -> C parses + typechecks" true
     (parse_check_passes
@@ -3395,6 +3416,8 @@ let type_syntax_sugar_tests = [
   Alcotest.test_case "xs[a:b]/[a:]/[:b]/[:] (#135 slice 2)"   `Quick test_slice_full_range;
   Alcotest.test_case "xs[0] index non-regressed (#135 sl.2)"  `Quick test_slice_index_not_regressed;
   Alcotest.test_case "effect E; + -> T / E (#135 slice 3)"    `Quick test_bare_effect_and_effect_row;
+  Alcotest.test_case "trait default body + ref self (#135 sl5)" `Quick test_trait_default_body;
+  Alcotest.test_case "trait sig + assoc non-regressed (#135 sl5)" `Quick test_trait_sig_and_assoc_not_regressed;
   Alcotest.test_case "(A, B) -> C (multi-arg arrow)"          `Quick test_multi_arg_arrow;
   Alcotest.test_case "(A, B) without arrow remains tuple"     `Quick test_tuple_type_still_works;
 ]