core: match guards through typing + interp + wasm (#67) (#74)

Closes #67.

## What changed

Match guards (`if e` after a pattern) now work end-to-end through
`ephapax-typing`, `ephapax-interp`, and `ephapax-wasm`. The grammar
already carries `MatchArm::guard: Option<Box<Expr>>` from #61 — this PR
replaces the `NotYetSupportedInCore` stub in typing, the `Unimplemented`
stub in interp, and adds the `i32.eqz + br_if` emission in wasm.

## Per-crate changes

### `ephapax-typing`
- `check_match` type-checks the guard under the arm's pattern bindings;
unified against `Bool` at the guard's span. Non-Bool guards return
`TypeMismatch`.
- `check_exhaustiveness` now skips guarded arms when building the
Maranget matrix — guards can refute at runtime so they don't contribute
to coverage. `Some(v) if v > 0 -> ... | None -> ...` is correctly
reported as non-exhaustive.

### `ephapax-interp`
- `eval_match` evaluates the guard after applying the pattern bindings.
`false` ⇒ restore bindings, continue to next arm. `true` ⇒ run body.
- Failed guards must not leak bindings — verified by a test where an
outer `v=999` survives a refuted `Some(v)` arm.

### `ephapax-wasm`
- `compile_match` emits the guard expression after the destructure.
`i32.eqz + br_if 0` falls through to the surrounding `$arm_i_fail`
block, reusing the per-arm fall-through machinery from #68.

### `ephapax-linear`
- No changes. The N-arm branch-agreement check from #65 already walks
`arm.guard`, so guarded-arm linearity is enforced automatically.

## Behaviour table

| Code | Outcome |
|---|---|
| `match x { Some(v) if v > 0 => v \| Some(v) => v \| None => 0 }` |
Typechecks; covers Some entirely via the second arm. |
| `match x { Some(v) if v > 0 => v \| None => 0 }` | `NonExhaustiveMatch
{ missing: "Some(_)" }`. |
| `Some(_) if 42 => ...` | `TypeMismatch` (guard is `I32`, expected
`Bool`). |
| `Some(7) if v > 0 => v` | Interp returns 7. |
| `Some(-3) if v > 0 => v \| _ => 0` | Interp returns 0 (guard falls
through). |

## Test plan

- [x] `cargo test -p ephapax-typing --lib` → 73 pass (was 70), +3 guard
tests.
- [x] `cargo test -p ephapax-interp --lib` → 17 pass (was 14), +3 guard
tests.
- [x] `cargo test -p ephapax-wasm --lib` → 79 pass (was 77), +2 guard
tests (validated via `wasmparser`).
- [x] Full workspace lib tests pass.
- [ ] CI green on all checks.

## Out of scope

- Or-patterns inside a single arm — separate feature.
- Guards containing `match` — should fall out for free; no targeted test
added.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Author: hyperpolymath

src/ephapax-interp/src/lib.rs

@@ -902,11 +902,6 @@ impl Interpreter {
     ) -> Result<Value, RuntimeError> {
         let scrut_val = self.eval(scrutinee)?;
         for arm in arms {
-            if arm.guard.is_some() {
-                return Err(RuntimeError::Unimplemented(
-                    "match guards not yet implemented in interpreter".into(),
-                ));
-            }
             let mut new_bindings: Vec<(Var, Value)> = Vec::new();
             if self.try_match_pattern(&scrut_val, &arm.pattern, &mut new_bindings)? {
                 let saved: Vec<(Var, Option<Value>, Option<bool>)> = new_bindings
@@ -922,6 +917,40 @@ impl Interpreter {
                 for (name, value) in new_bindings {
                     self.env.extend(name, value);
                 }
+
+                // Evaluate guard (if any) under the arm's pattern
+                // bindings. If it returns `false`, restore prior
+                // bindings and try the next arm.
+                let guard_ok = match &arm.guard {
+                    None => true,
+                    Some(g) => match self.eval(g)? {
+                        Value::Bool(b) => b,
+                        other => {
+                            return Err(RuntimeError::TypeError {
+                                expected: "Bool".into(),
+                                found: other.type_name().into(),
+                            });
+                        }
+                    },
+                };
+
+                if !guard_ok {
+                    for (name, prev_val, prev_consumed) in saved.iter().rev() {
+                        match prev_val {
+                            Some(v) => {
+                                self.env.bindings.insert(name.clone(), v.clone());
+                                if let Some(c) = prev_consumed {
+                                    self.env.consumed.insert(name.clone(), *c);
+                                }
+                            }
+                            None => {
+                                self.env.remove(name);
+                            }
+                        }
+                    }
+                    continue;
+                }
+
                 let result = self.eval(&arm.body)?;
                 for (name, prev_val, prev_consumed) in saved.iter().rev() {
                     match prev_val {
@@ -1701,4 +1730,122 @@ mod tests {
         // with the typing test module pattern).
         let _ = Visibility::Private;
     }
+
+    // ----- Match guards (#67) -----
+
+    /// Guard reads pattern-bound var and evaluates to `true` → arm body
+    /// runs. `match Some(7) { Some(v) if v > 0 => v | _ => 0 }` = 7.
+    #[test]
+    fn test_eval_match_guard_pass() {
+        let mut interp = Interpreter::new();
+        interp.load_module(&option_module());
+        let scrut = dummy_expr(ExprKind::Inr {
+            ty: Ty::Base(BaseTy::Unit),
+            value: Box::new(lit_i32_expr(7)),
+        });
+        let guard = dummy_expr(ExprKind::BinOp {
+            op: BinOp::Gt,
+            left: Box::new(dummy_expr(ExprKind::Var("v".into()))),
+            right: Box::new(lit_i32_expr(0)),
+        });
+        let expr = dummy_expr(ExprKind::Match {
+            scrutinee: Box::new(scrut),
+            arms: vec![
+                MatchArm {
+                    pattern: P::Constructor {
+                        ctor: "Some".into(),
+                        args: vec![P::Var("v".into())],
+                    },
+                    guard: Some(Box::new(guard)),
+                    body: dummy_expr(ExprKind::Var("v".into())),
+                },
+                MatchArm {
+                    pattern: P::Wildcard,
+                    guard: None,
+                    body: lit_i32_expr(0),
+                },
+            ],
+        });
+        let result = interp.eval(&expr).expect("guarded match should evaluate");
+        assert!(matches!(result, Value::I32(7)), "got {:?}", result);
+    }
+
+    /// Guard evaluates to `false` → arm is skipped, next arm runs.
+    /// `match Some(-3) { Some(v) if v > 0 => v | _ => 0 }` = 0.
+    #[test]
+    fn test_eval_match_guard_fail_falls_through() {
+        let mut interp = Interpreter::new();
+        interp.load_module(&option_module());
+        let scrut = dummy_expr(ExprKind::Inr {
+            ty: Ty::Base(BaseTy::Unit),
+            value: Box::new(lit_i32_expr(-3)),
+        });
+        let guard = dummy_expr(ExprKind::BinOp {
+            op: BinOp::Gt,
+            left: Box::new(dummy_expr(ExprKind::Var("v".into()))),
+            right: Box::new(lit_i32_expr(0)),
+        });
+        let expr = dummy_expr(ExprKind::Match {
+            scrutinee: Box::new(scrut),
+            arms: vec![
+                MatchArm {
+                    pattern: P::Constructor {
+                        ctor: "Some".into(),
+                        args: vec![P::Var("v".into())],
+                    },
+                    guard: Some(Box::new(guard)),
+                    body: dummy_expr(ExprKind::Var("v".into())),
+                },
+                MatchArm {
+                    pattern: P::Wildcard,
+                    guard: None,
+                    body: lit_i32_expr(0),
+                },
+            ],
+        });
+        let result = interp.eval(&expr).expect("guard fall-through should evaluate");
+        assert!(matches!(result, Value::I32(0)), "got {:?}", result);
+    }
+
+    /// Failed guard must not leak its bindings into subsequent arms.
+    /// `v` is pre-bound in the enclosing scope; the first arm tries to
+    /// rebind `v` and its guard fails — the second arm must see the
+    /// original `v`.
+    #[test]
+    fn test_eval_match_failed_guard_does_not_leak_bindings() {
+        let mut interp = Interpreter::new();
+        interp.load_module(&option_module());
+        interp.env.extend("v".into(), Value::I32(999));
+
+        let scrut = dummy_expr(ExprKind::Inr {
+            ty: Ty::Base(BaseTy::Unit),
+            value: Box::new(lit_i32_expr(-1)),
+        });
+        let guard = dummy_expr(ExprKind::BinOp {
+            op: BinOp::Gt,
+            left: Box::new(dummy_expr(ExprKind::Var("v".into()))),
+            right: Box::new(lit_i32_expr(0)),
+        });
+        let expr = dummy_expr(ExprKind::Match {
+            scrutinee: Box::new(scrut),
+            arms: vec![
+                MatchArm {
+                    pattern: P::Constructor {
+                        ctor: "Some".into(),
+                        args: vec![P::Var("v".into())],
+                    },
+                    guard: Some(Box::new(guard)),
+                    body: dummy_expr(ExprKind::Var("v".into())),
+                },
+                MatchArm {
+                    pattern: P::Wildcard,
+                    guard: None,
+                    // Second arm reads the OUTER `v` (still 999).
+                    body: dummy_expr(ExprKind::Var("v".into())),
+                },
+            ],
+        });
+        let result = interp.eval(&expr).expect("fall-through should evaluate");
+        assert!(matches!(result, Value::I32(999)), "got {:?}", result);
+    }
 }

src/ephapax-typing/src/lib.rs

@@ -1473,8 +1473,9 @@ impl TypeChecker {
 
             let bound = self.check_pattern(s, &arm.pattern, &scrutinee_ty)?;
 
-            if arm.guard.is_some() {
-                return Err(self.at(s, TypeError::NotYetSupportedInCore("match guards")));
+            if let Some(guard) = &arm.guard {
+                let guard_ty = self.check(guard)?;
+                self.unify(guard.span, &Ty::Base(BaseTy::Bool), &guard_ty)?;
             }
 
             let body_ty = self.check(&arm.body)?;
@@ -1651,8 +1652,12 @@ impl TypeChecker {
         arms: &[MatchArm],
         _scrutinee_ty: &Ty,
     ) -> Result<(), SpannedTypeError> {
+        // Guarded arms can refute at runtime — they don't contribute
+        // to coverage. Standard treatment: build the exhaustiveness
+        // matrix from arms whose `guard.is_none()`.
         let matrix: Vec<Vec<Pattern>> = arms
             .iter()
+            .filter(|a| a.guard.is_none())
             .map(|a| vec![a.pattern.clone()])
             .collect();
         if let Some(witness) = is_useful(&matrix, 1, &self.data_registry) {
@@ -4331,4 +4336,158 @@ mod tests {
             other => panic!("expected NonExhaustiveMatch, got {other:?}"),
         }
     }
+
+    // ----- Match guards (#67) -----
+
+    /// `match x { Some(v) if v > 0 => 1 | Some(v) => 2 | None => 0 }`
+    /// — guard uses bound pattern var; non-guarded `Some(v)` covers
+    /// the remaining case, so exhaustiveness holds.
+    #[test]
+    fn test_match_guard_typechecks_with_pattern_binding() {
+        let scrut = dummy_expr(ExprKind::Inr {
+            ty: Ty::Base(BaseTy::Unit),
+            value: Box::new(lit_i32(5)),
+        });
+        let arms = vec![
+            MatchArm {
+                pattern: P::Constructor {
+                    ctor: "Some".into(),
+                    args: vec![P::Var("v".into())],
+                },
+                guard: Some(Box::new(dummy_expr(ExprKind::BinOp {
+                    op: BinOp::Gt,
+                    left: Box::new(dummy_expr(ExprKind::Var("v".into()))),
+                    right: Box::new(lit_i32(0)),
+                }))),
+                body: lit_i32(1),
+            },
+            MatchArm {
+                pattern: P::Constructor {
+                    ctor: "Some".into(),
+                    args: vec![P::Var("v".into())],
+                },
+                guard: None,
+                body: dummy_expr(ExprKind::Var("v".into())),
+            },
+            MatchArm {
+                pattern: P::Constructor {
+                    ctor: "None".into(),
+                    args: vec![],
+                },
+                guard: None,
+                body: lit_i32(0),
+            },
+        ];
+        let body = dummy_expr(ExprKind::Match {
+            scrutinee: Box::new(scrut),
+            arms,
+        });
+        let module = Module {
+            name: "t".into(),
+            imports: vec![],
+            decls: vec![
+                option_data_decl(),
+                fn_decl("f", vec![], Ty::Base(BaseTy::I32), body),
+            ],
+        };
+        type_check_module(&module).expect("guard binding to v: I32 should typecheck");
+    }
+
+    /// A guard whose expression is not `Bool` is rejected with a
+    /// type mismatch.
+    #[test]
+    fn test_match_guard_non_bool_rejected() {
+        let scrut = dummy_expr(ExprKind::Inr {
+            ty: Ty::Base(BaseTy::Unit),
+            value: Box::new(lit_i32(5)),
+        });
+        let arms = vec![
+            MatchArm {
+                pattern: P::Constructor {
+                    ctor: "Some".into(),
+                    args: vec![P::Var("v".into())],
+                },
+                guard: Some(Box::new(lit_i32(42))), // I32, not Bool
+                body: lit_i32(1),
+            },
+            MatchArm {
+                pattern: P::Wildcard,
+                guard: None,
+                body: lit_i32(0),
+            },
+        ];
+        let body = dummy_expr(ExprKind::Match {
+            scrutinee: Box::new(scrut),
+            arms,
+        });
+        let module = Module {
+            name: "t".into(),
+            imports: vec![],
+            decls: vec![
+                option_data_decl(),
+                fn_decl("f", vec![], Ty::Base(BaseTy::I32), body),
+            ],
+        };
+        let err = type_check_module(&module).unwrap_err();
+        assert!(
+            matches!(err.error, TypeError::TypeMismatch { .. }),
+            "expected TypeMismatch for non-Bool guard, got {:?}",
+            err.error
+        );
+    }
+
+    /// A guarded arm cannot make the match exhaustive on its own —
+    /// its guard could refute at runtime. `Some(v) if v > 0` followed
+    /// by `None` is missing the `Some` non-positive case.
+    #[test]
+    fn test_match_guarded_arm_not_counted_for_exhaustiveness() {
+        let scrut = dummy_expr(ExprKind::Inr {
+            ty: Ty::Base(BaseTy::Unit),
+            value: Box::new(lit_i32(5)),
+        });
+        let arms = vec![
+            MatchArm {
+                pattern: P::Constructor {
+                    ctor: "Some".into(),
+                    args: vec![P::Var("v".into())],
+                },
+                guard: Some(Box::new(dummy_expr(ExprKind::BinOp {
+                    op: BinOp::Gt,
+                    left: Box::new(dummy_expr(ExprKind::Var("v".into()))),
+                    right: Box::new(lit_i32(0)),
+                }))),
+                body: lit_i32(1),
+            },
+            MatchArm {
+                pattern: P::Constructor {
+                    ctor: "None".into(),
+                    args: vec![],
+                },
+                guard: None,
+                body: lit_i32(0),
+            },
+        ];
+        let body = dummy_expr(ExprKind::Match {
+            scrutinee: Box::new(scrut),
+            arms,
+        });
+        let module = Module {
+            name: "t".into(),
+            imports: vec![],
+            decls: vec![
+                option_data_decl(),
+                fn_decl("f", vec![], Ty::Base(BaseTy::I32), body),
+            ],
+        };
+        let err = type_check_module(&module).unwrap_err();
+        match err.error {
+            TypeError::NonExhaustiveMatch { missing } => {
+                assert!(
+                    missing.as_str().contains("Some"),
+                    "expected witness mentioning Some, got {missing}"
+                );
+            }
+            other => panic!("expected NonExhaustiveMatch, got {other:?}"),
+        }
+    }
 }