haskell: Phase 13 — where-clauses in instance bodies (desugar fix, +4 tests)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

plans/haskell-completeness.md

@@ -220,7 +220,7 @@ No OCaml changes are needed. The view type is fully representable as an SX dict.
 
 ### Phase 13 — `where` in typeclass instances + default methods
 
-- [ ] Verify `where`-clauses in `instance` bodies desugar correctly. The
+- [x] Verify `where`-clauses in `instance` bodies desugar correctly. The
   `hk-bind-decls!` instance arm must call the same where-lifting logic as
   top-level function clauses. Write a targeted test to confirm.
 - [ ] Class declarations may include default method implementations. Parser:
@@ -307,6 +307,18 @@ No OCaml changes are needed. The view type is fully representable as an SX dict.
 
 _Newest first._
 
+**2026-05-07** — Phase 13 `where`-clauses in `instance` bodies:
+- Bug discovered: `hk-desugar` didn't recurse into `instance-decl` method
+  bodies, so a `where`-form in an instance method survived to eval and hit
+  `eval: unknown node tag 'where'`. Fix: added an `instance-decl` case to
+  the desugarer that maps `hk-desugar` over the method-decls list. The
+  existing `fun-clause` branch then desugars each method body, including
+  the where → let lifting.
+- 4 tests in new `tests/instance-where.sx`: where-helper with literal
+  pattern matching, references reused multiple times, and multi-binding
+  where. Verified no regression in class.sx (14/14), deriving.sx (15/15),
+  desugar.sx (15/15).
+
 **2026-05-07** — Phase 12 conformance: uniquewords.hs (4/4) + setops.hs (8/8) → Phase 12 complete:
 - `program-uniquewords.sx`: `foldl Set.insert` over a word list, then check
   `Set.size`/`member`. 4/4.