ERR/E1.5: catch sema (pure-failable slice) + error-set match subjects

`expr catch [e] BODY` consumes a failable's error inline. Pure-failable slice
(value-carrying `-> (T, !)` catch deferred to E2's tuple ABI).

- lowerExpr `.catch_expr` -> lowerCatch; inferExprType `.catch_expr` ->
  operand's success type (void for pure-failable).
- lowerCatch: operand must be failable (else "catch requires a failable
  expression"); pure-failable LHS only (value-carrying bails to E2). Eval
  operand -> err tag; condBr to handle (error) / merge (success). In handle:
  child scope binds `e` to the tag (typed as the error set), lower body
  (block or expr); if the body didn't diverge, br merge. Result is void.
  `catch` needs no failable enclosing function — it handles the error locally.
- All four body forms work: block, no-binding `catch { }`, bare-expr, and
  the match-body `catch e == { case ... }`. Re-raise (`raise e`) and diverging
  bodies (`return`) rely on E1.3 / E1.4c.

Also: lowerMatch now supports error-set subjects — `case .X` resolves to the
global tag id (was the arm index, dispatching wrong), and the switch operand
is the error-set value (its u32 tag) directly rather than via enumTag. This
is what the catch match-body form (and a plain `if e == { case .X }`) needs.

Tests: examples/226-catch.sx (block / no-binding / match-body / re-raise /
diverging body / success-skip; exit 18), examples/227-catch-rejections.sx
(operand-not-failable; exit 1). Gates: zig build, zig build test,
265/265 examples.

examples/226-catch.sx

@@ -0,0 +1,64 @@
+// `catch` on a pure-failable LHS (ERR step E1.5). `expr catch [e] BODY`
+// consumes the error inline: on failure it binds the tag to `e` (optional)
+// and runs BODY; on success the result is void (a `-> !` LHS has no success
+// value). BODY may diverge (`return` / `raise` — typed `noreturn`, E1.4c) or
+// fall through. `catch` needs no failable *enclosing* function — it handles
+// the error locally. All four body forms appear below: block, no-binding
+// block, match-body (`== { case ... }`), and the selective handle + re-raise
+// pattern. Value-carrying `-> (T, !)` catch (binding the success value) lands
+// with the tuple ABI in E2.
+
+#import "modules/std.sx";
+
+E :: error { Bad, Empty }
+
+must :: (n: s32) -> !E {
+    if n < 0  { raise error.Bad; }
+    if n == 0 { raise error.Empty; }
+    return;
+}
+
+// Diverging body — returns from `classify` on error.
+classify :: (n: s32) -> s32 {
+    must(n) catch e {
+        if e == error.Bad   { return 1; }
+        if e == error.Empty { return 2; }
+        return 9;
+    };
+    return 0;                               // must(n) succeeded
+}
+
+// Match-body form — sugar for `catch e { if e == { case ... } }`.
+mclassify :: (n: s32) -> s32 {
+    must(n) catch e == {
+        case .Bad:   return 11;
+        case .Empty: return 22;
+        else:        return 99;
+    };
+    return 0;
+}
+
+// Selective handle + re-raise (failable enclosing fn; `raise e` is the
+// variable form). Swallows Bad → success; re-raises everything else.
+handle_some :: (n: s32) -> !E {
+    must(n) catch e {
+        if e == error.Bad { return; }       // swallow → success
+        raise e;                            // re-raise the rest
+    };
+    return;
+}
+
+main :: () -> s32 {
+    r : s32 = 0;
+    must(-1) catch e { if e == error.Bad { r = r + 1; } };  // Bad → +1
+    must(5)  catch { r = r + 100; };                         // success → body skipped
+    r = r + classify(0);                                     // Empty → 2
+    r = r + classify(8);                                     // success → 0
+    he := handle_some(0);                                    // Empty re-raised
+    if he == error.Empty { r = r + 4; }                      // +4
+    hb := handle_some(-1);                                   // Bad swallowed → success
+    if hb == error.Bad { r = r + 50; }                       // not taken
+    r = r + mclassify(-1);                                   // Bad → 11
+    print("catch result: {}\n", r);                          // 1+2+4+11 = 18
+    return r;
+}

examples/227-catch-rejections.sx

@@ -0,0 +1,13 @@
+// `catch` rejection (ERR step E1.5): the operand must be failable. Unlike
+// `try` / `raise`, `catch` needs no failable enclosing function — it consumes
+// the error locally — so the only stable rejection is a non-failable operand.
+// The positive cases live in `examples/226-catch.sx`.
+
+#import "modules/std.sx";
+
+plain :: () -> s32 { return 0; }
+
+main :: () -> s32 {
+    plain() catch e { return 1; };   // error: operand has type s32 (not failable)
+    return 0;
+}

src/ir/lower.zig

@@ -2571,6 +2571,7 @@ pub const Lowering = struct {
             },
 
             .try_expr => |te| self.lowerTry(te.operand, node.span),
+            .catch_expr => |ce| self.lowerCatch(&ce, node.span),
             else => self.emitError("unknown_expr", node.span),
         };
     }
@@ -3707,6 +3708,15 @@ pub const Lowering = struct {
             }
             break :blk false;
         };
+        // An error-set subject (`catch e == { case .X: ... }` / `if e == { ... }`):
+        // the value IS its u32 tag id, and `case .X` matches the global tag id
+        // of `X`. Used by ERR E1.5's catch match-body form.
+        const is_error_set_match = blk: {
+            if (!subject_ty.isBuiltin()) {
+                break :blk self.module.types.get(subject_ty) == .error_set;
+            }
+            break :blk false;
+        };
 
         // Determine if the match produces a value (has non-void arms)
         // For type-category matches (inside any_to_string), only produce value when force_block_value
@@ -3823,6 +3833,9 @@ pub const Lowering = struct {
                                 const ty_name = self.formatTypeName(subject_ty);
                                 diags.addFmt(.err, pat.span, "no variant '{s}' on type '{s}'", .{ pat_name, ty_name });
                             }
+                        } else if (ty_info == .error_set) {
+                            // `case .X` matches the global tag id of `X`.
+                            break :blk @intCast(self.module.types.internTag(pat_name));
                         }
                     }
                     break :blk @intCast(i);
@@ -3848,7 +3861,7 @@ pub const Lowering = struct {
                 break :tag_blk self.builder.emit(.{ .unbox_any = .{ .operand = subject } }, .s64);
             }
             break :tag_blk subject;
-        } else if (is_optional_match) self.builder.emit(.{ .optional_has_value = .{ .operand = subject } }, .bool) else blk: {
+        } else if (is_optional_match) self.builder.emit(.{ .optional_has_value = .{ .operand = subject } }, .bool) else if (is_error_set_match) subject else blk: {
             // Determine actual tag type from union info (e.g. u32 for SDL_Event)
             const tag_ty: TypeId = tt: {
                 if (!subject_ty.isBuiltin()) {
@@ -13584,6 +13597,16 @@ pub const Lowering = struct {
                 if (info == .tuple and info.tuple.fields.len == 2) break :blk info.tuple.fields[0];
                 break :blk op_ty;
             },
+            // `expr catch ...` strips the error channel → the success type
+            // (void for a pure-failable LHS; the value part for value-carrying).
+            .catch_expr => |ce| blk: {
+                const op_ty = self.inferExprType(ce.operand);
+                const channel = self.errorChannelOf(op_ty) orelse break :blk .unresolved;
+                if (op_ty == channel) break :blk .void;
+                const info = self.module.types.get(op_ty);
+                if (info == .tuple and info.tuple.fields.len == 2) break :blk info.tuple.fields[0];
+                break :blk op_ty;
+            },
             .if_expr => |ie| {
                 // If-else types as its branches' unified type. A `noreturn`
                 // branch (one that diverges — `return` / `raise` / `break` /
@@ -15241,6 +15264,65 @@ pub const Lowering = struct {
         }
     }
 
+    /// `expr catch [e] BODY` — inline failure handler (ERR step E1.5,
+    /// pure-failable slice). Evaluates `expr`; on failure, binds the tag to
+    /// `e` (if present) and runs BODY; on success, the value is `void` (a
+    /// pure-failable LHS has no success value). BODY either diverges (via
+    /// `noreturn` — E1.4c) or falls through. `catch` consumes the error
+    /// locally, so — unlike `try` / `raise` — it needs no failable *enclosing*
+    /// function. Value-carrying LHS (binding the success value / a
+    /// value-producing body unifying with the success tuple) needs the
+    /// error-channel tuple ABI and lands in E2 — bail loudly here.
+    fn lowerCatch(self: *Lowering, ce: *const ast.CatchExpr, span: ast.Span) Ref {
+        const op_ty = self.inferExprType(ce.operand);
+        const err_set = self.errorChannelOf(op_ty) orelse {
+            if (self.diagnostics) |diags| {
+                diags.addFmt(.err, span, "`catch` requires a failable expression; operand has type '{s}'", .{self.formatTypeName(op_ty)});
+            }
+            return self.builder.constInt(0, .void);
+        };
+        if (op_ty != err_set) {
+            if (self.diagnostics) |diags| {
+                diags.addFmt(.err, span, "`catch` on a value-carrying failable (`-> (T..., !)`) is not yet lowered — pending the error-channel tuple ABI (ERR E2)", .{});
+            }
+            return self.builder.constInt(0, .void);
+        }
+
+        // Evaluate the operand → the error tag (0 = success), then branch.
+        const err_val = self.lowerExpr(ce.operand);
+        const err_ty = self.builder.getRefType(err_val);
+        const zero = self.builder.constInt(0, err_ty);
+        const is_err = self.builder.emit(.{ .cmp_ne = .{ .lhs = err_val, .rhs = zero } }, .bool);
+
+        const handle_bb = self.freshBlock("catch.handle");
+        const merge_bb = self.freshBlock("catch.merge");
+        // On success (err == 0) jump straight to the merge; on error, handle.
+        self.builder.condBr(is_err, handle_bb, &.{}, merge_bb, &.{});
+
+        // Handle: bind the tag (typed as the error set) and run the body in a
+        // child scope. The body diverges (terminates the block) or falls
+        // through to the merge.
+        self.builder.switchToBlock(handle_bb);
+        var handle_scope = Scope.init(self.alloc, self.scope);
+        const saved_scope = self.scope;
+        self.scope = &handle_scope;
+        if (ce.binding) |name| {
+            handle_scope.put(name, .{ .ref = err_val, .ty = err_set, .is_alloca = false });
+        }
+        if (ce.body.data == .block) {
+            self.lowerBlock(ce.body);
+        } else {
+            _ = self.lowerExpr(ce.body);
+        }
+        self.scope = saved_scope;
+        handle_scope.deinit();
+        if (!self.currentBlockHasTerminator()) self.builder.br(merge_bb, &.{});
+
+        // Merge (success path + non-diverging handle). Pure-failable → void.
+        self.builder.switchToBlock(merge_bb);
+        return self.builder.constInt(0, .void);
+    }
+
     fn bailTry(self: *Lowering, span: ast.Span, comptime what: []const u8) Ref {
         if (self.diagnostics) |diags| {
             diags.addFmt(.err, span, "`try` with " ++ what ++ " is not yet lowered — pending the error-channel tuple ABI (ERR E2)", .{});

tests/expected/226-catch.exit

@@ -0,0 +1 @@
+18

tests/expected/226-catch.txt

@@ -0,0 +1 @@
+catch result: 18

tests/expected/227-catch-rejections.exit

@@ -0,0 +1 @@
+1

tests/expected/227-catch-rejections.txt

@@ -0,0 +1,5 @@
+error: `catch` requires a failable expression; operand has type 's32'
+  --> /Users/agra/projects/sx/examples/227-catch-rejections.sx:11:5
+   |
+11 |     plain() catch e { return 1; };   // error: operand has type s32 (not failable)
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^