diff --git a/examples/1040-errors-failable-closure-composition.sx b/examples/1040-errors-failable-closure-composition.sx
new file mode 100644
index 0000000..b6ef790
--- /dev/null
+++ b/examples/1040-errors-failable-closure-composition.sx
@@ -0,0 +1,34 @@
+// Failable closure composition (ERR E5.1): a closure LITERAL passed as a
+// function-type argument and called inside the callee. Covers a bare failable
+// fn-type param (`cb: (T) -> (U, !)`), the idiomatic `Closure(...)` param
+// (try-propagated), and ∅-widening of a NON-failable closure literal into a
+// failable slot (the generated adapter wraps the value into `{value, 0}`).
+//
+// NOTE: the adapter is generated when the closure LITERAL flows directly into
+// the bare-fn slot. Passing a pre-bound closure *variable* into a bare-fn slot
+// is a separate coercion-site path, not yet handled — see CHECKPOINT-ERR.
+
+#import "modules/std.sx";
+
+E :: error { Neg }
+
+bare  :: (cb: (s64) -> (s64, !E), n: s64) -> s64 { return cb(n) catch e -1; }
+chain :: (cb: Closure(s64) -> (s64, !E), n: s64) -> (s64, !E) { return try cb(n); }
+
+dbl :: (x: s64) -> (s64, !E) { if x < 0 { raise error.Neg; } return x * 2; }
+
+main :: () -> s32 {
+    // failable closure literal through a bare fn-type param (matching ABI)
+    print("bare ok={} err={}\n",
+        bare(closure((x: s64) -> (s64, !E) { if x < 0 { raise error.Neg; } return x * 2; }), 5),
+        bare(closure((x: s64) -> (s64, !E) => x * 2), -1));                 // ok=10; err: arrow never raises → cb(-1) = -2
+
+    // Closure(...) param, try-propagated, then caught at the call site
+    print("chain ok={} err={}\n",
+        chain(closure((x: s64) -> (s64, !E) => x + 6), 4) catch e 0,        // 10
+        chain(closure((x: s64) -> (s64, !E) { raise error.Neg; }), 1) catch e 0); // 0
+
+    // NON-failable closure literal widened into the failable bare slot
+    print("widen={}\n", bare(closure((x: s64) -> s64 => x + 1), 9));        // 10
+    return 0;
+}
diff --git a/examples/expected/1040-errors-failable-closure-composition.exit b/examples/expected/1040-errors-failable-closure-composition.exit
new file mode 100644
index 0000000..573541a
--- /dev/null
+++ b/examples/expected/1040-errors-failable-closure-composition.exit
@@ -0,0 +1 @@
+0
diff --git a/examples/expected/1040-errors-failable-closure-composition.stderr b/examples/expected/1040-errors-failable-closure-composition.stderr
new file mode 100644
index 0000000..8b13789
--- /dev/null
+++ b/examples/expected/1040-errors-failable-closure-composition.stderr
@@ -0,0 +1 @@
+
diff --git a/examples/expected/1040-errors-failable-closure-composition.stdout b/examples/expected/1040-errors-failable-closure-composition.stdout
new file mode 100644
index 0000000..46c0eee
--- /dev/null
+++ b/examples/expected/1040-errors-failable-closure-composition.stdout
@@ -0,0 +1,3 @@
+bare ok=10 err=-2
+chain ok=10 err=0
+widen=10
diff --git a/src/ir/lower.zig b/src/ir/lower.zig
index 6c1a507..2adf685 100644
--- a/src/ir/lower.zig
+++ b/src/ir/lower.zig
@@ -7891,10 +7891,13 @@ pub const Lowering = struct {
         if (self.target_type) |tt| {
             if (!tt.isBuiltin() and self.module.types.get(tt) == .function) {
                 const slot_ret = self.module.types.get(tt).function.ret;
+                const widen_ok = self.errorChannelOf(slot_ret) != null and self.errorChannelOf(ret_ty) == null and self.failableSuccessType(slot_ret) == ret_ty;
                 if (capture_list.len > 0) {
                     if (self.diagnostics) |d| d.addFmt(.err, lam.body.span, "a capturing closure cannot be passed as a bare function pointer; declare the parameter type as `Closure(...)` so its environment is carried", .{});
-                } else if (ret_ty == slot_ret) {
-                    const adapter = self.createClosureToBareFnAdapter(func_id, self.module.types.get(tt).function);
+                } else if (ret_ty == slot_ret or widen_ok) {
+                    // Matching ABI, or a non-failable closure widening into a
+                    // failable slot (∅ ⊆ slot set) — the adapter wraps {value, 0}.
+                    const adapter = self.createClosureToBareFnAdapter(func_id, self.module.types.get(tt).function, ret_ty, lam.body.span);
                     return self.builder.emit(.{ .func_ref = adapter }, tt);
                 } else if (self.errorChannelOf(ret_ty) != null and self.errorChannelOf(slot_ret) == null) {
                     if (self.diagnostics) |d| d.addFmt(.err, lam.body.span, "failable closure cannot be assigned to a non-failable function-type slot; foreign code can't observe the error channel — handle the error in a wrapper closure that absorbs it", .{});
@@ -8030,7 +8033,12 @@ pub const Lowering = struct {
     /// (`[ctx?] + user-params`) and forwards to the closure fn with a null env.
     /// Only sound for capture-free closures (a null env is correct iff the body
     /// reads no captures); the caller rejects capturing closures.
-    fn createClosureToBareFnAdapter(self: *Lowering, closure_func_id: FuncId, fn_info: types.TypeInfo.FunctionInfo) FuncId {
+    ///
+    /// When `closure_ret` differs from `fn_info.ret`, this is the ∅-widening
+    /// case (a non-failable closure into a failable slot): the closure returns
+    /// the success value and the adapter wraps it into the slot's `{value, 0}`
+    /// failable tuple (ERR E5.1 non-failable→failable widening).
+    fn createClosureToBareFnAdapter(self: *Lowering, closure_func_id: FuncId, fn_info: types.TypeInfo.FunctionInfo, closure_ret: TypeId, span: ast.Span) FuncId {
         var params = std.ArrayList(inst_mod.Function.Param).empty;
         defer params.deinit(self.alloc);
         const void_ptr_ty = self.module.types.ptrTo(.void);
@@ -8074,11 +8082,17 @@ pub const Lowering = struct {
             call_args.append(self.alloc, Ref.fromIndex(@intCast(ctx_slots + i))) catch unreachable;
         }
         const owned_args = self.alloc.dupe(Ref, call_args.items) catch unreachable;
-        const result = self.builder.emit(.{ .call = .{ .callee = closure_func_id, .args = owned_args } }, fn_info.ret);
-        if (fn_info.ret != .void) {
-            self.builder.ret(result, fn_info.ret);
+        const result = self.builder.emit(.{ .call = .{ .callee = closure_func_id, .args = owned_args } }, closure_ret);
+        if (closure_ret == fn_info.ret) {
+            if (fn_info.ret != .void) {
+                self.builder.ret(result, fn_info.ret);
+            } else {
+                self.builder.retVoid();
+            }
         } else {
-            self.builder.retVoid();
+            // ∅-widening: closure returns the success value; wrap `{value, 0}`
+            // into the slot's failable tuple.
+            self.lowerFailableSuccessReturn(result, fn_info.ret, span);
         }
         self.builder.finalize();
 
@@ -14190,12 +14204,16 @@ pub const Lowering = struct {
                         if (fd.return_type) |rt| return self.resolveType(rt);
                         return .void;
                     }
-                    // Check if callee is a local closure variable — extract return type
+                    // Check if callee is a local closure / function-type variable
+                    // (e.g. a `cb: Closure(...) -> R` or bare `cb: (T) -> R`
+                    // parameter) — extract its declared return type so `try` /
+                    // `catch` on the call see the (possibly failable) result.
                     if (self.scope) |scope| {
                         if (scope.lookup(bare_name)) |binding| {
                             if (!binding.ty.isBuiltin()) {
                                 const ti = self.module.types.get(binding.ty);
                                 if (ti == .closure) return ti.closure.ret;
+                                if (ti == .function) return ti.function.ret;
                             }
                         }
                     }