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-rw-r--r--tvix/eval/src/compiler/mod.rs12
-rw-r--r--tvix/eval/src/tests/tvix_tests/eval-okay-deferred-unary-formals.exp1
-rw-r--r--tvix/eval/src/tests/tvix_tests/eval-okay-deferred-unary-formals.nix6
3 files changed, 18 insertions, 1 deletions
diff --git a/tvix/eval/src/compiler/mod.rs b/tvix/eval/src/compiler/mod.rs
index 5460514ba1ed..39b706cb487a 100644
--- a/tvix/eval/src/compiler/mod.rs
+++ b/tvix/eval/src/compiler/mod.rs
@@ -937,7 +937,17 @@ impl Compiler<'_> {
                 let jump_over_default = self.push_op(OpCode::OpJump(JumpOffset(0)), &default_expr);
 
                 self.patch_jump(jump_to_default);
-                self.compile(idx, default_expr);
+
+                // Thunk the default expression, but only if it is something
+                // other than an identifier.
+                if let ast::Expr::Ident(_) = &default_expr {
+                    self.compile(idx, default_expr);
+                } else {
+                    self.thunk(idx, &self.span_for(&default_expr), move |c, s| {
+                        c.compile(s, default_expr)
+                    });
+                }
+
                 self.patch_jump(jump_over_default);
             } else {
                 self.push_op(OpCode::OpAttrsSelect, &entry.ident().unwrap());
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-deferred-unary-formals.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-deferred-unary-formals.exp
new file mode 100644
index 000000000000..5993db7ccc5a
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-deferred-unary-formals.exp
@@ -0,0 +1 @@
+[ false -2 ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-deferred-unary-formals.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-deferred-unary-formals.nix
new file mode 100644
index 000000000000..1fbb3e853af4
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-deferred-unary-formals.nix
@@ -0,0 +1,6 @@
+# Application of unary operators on deferred formals arguments (via
+# defaulting), see also b/255.
+[
+  (({ b ? !a, a }: b) { a = true; })
+  (({ b ? -a, a }: b) { a = 2; })
+]