From b319186a684959a5b9d63906d9c5a11fe48fd1fe Mon Sep 17 00:00:00 2001
From: Chris Tate <chris@ctate.dev>
Date: Thu, 29 Oct 2026 03:01:39 -0500
Subject: [PATCH] Count the arguments a replaced stdio write receives

A JavaScript function assigned to process.stdout.write or
process.stderr.write takes its parameters from the call, not from Node's
overloads, but the lowering returned before giving the function its
arguments slot. `arguments.length` then folded to the declared parameter
count (3 for `function (chunk, encoding, callback)` called with one
argument, 0 for a function without parameters), and reading
`arguments[i]` was refused at run time.

The replacement now receives the call's arguments, so `arguments` holds
what the caller passed, as in Node.
---
 .../src/frontend/lowering/call-signatures.ts  |  3 ++
 ...stdio-write-replacement-arguments.cjs.json |  6 +++
 .../stdio-write-replacement-arguments.cjs     | 43 +++++++++++++++++++
 ...stdio-write-replacement-arguments.cjs.json |  1 +
 4 files changed, 53 insertions(+)
 create mode 100644 packages/compiler/test/ts7/baselines/order-parity/tests/corpus/stdio-write-replacement-arguments.cjs.json
 create mode 100644 tests/corpus/stdio-write-replacement-arguments.cjs
 create mode 100644 tests/harness/corpus-costs/stdio-write-replacement-arguments.cjs.json

diff --git a/packages/compiler/src/frontend/lowering/call-signatures.ts b/packages/compiler/src/frontend/lowering/call-signatures.ts
index 45329d077..783082288 100644
--- a/packages/compiler/src/frontend/lowering/call-signatures.ts
+++ b/packages/compiler/src/frontend/lowering/call-signatures.ts
@@ -1062,6 +1062,9 @@ export function lambdaSignature(
         type: DYN,
         mode: "required",
       }));
+      // `arguments` holds the call's own tuple: a console line passes one
+      // or two arguments whatever the replacement declares.
+      appendArgumentsParameter(lowerer, node, shapes, "arguments");
       return { shapes, funcType: funcTypeFromParamShapes(shapes, DYN) };
     }
   }
diff --git a/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/stdio-write-replacement-arguments.cjs.json b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/stdio-write-replacement-arguments.cjs.json
new file mode 100644
index 000000000..23315d78e
--- /dev/null
+++ b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/stdio-write-replacement-arguments.cjs.json
@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/stdio-write-replacement-arguments.cjs"
+ ],
+ "diags": []
+}
diff --git a/tests/corpus/stdio-write-replacement-arguments.cjs b/tests/corpus/stdio-write-replacement-arguments.cjs
new file mode 100644
index 000000000..c33a78210
--- /dev/null
+++ b/tests/corpus/stdio-write-replacement-arguments.cjs
@@ -0,0 +1,43 @@
+// A replaced process.stdout.write or process.stderr.write sees the call's own
+// arguments: arguments.length is the number the caller passed, whatever the
+// replacement declares.
+const seen = [];
+const originalOut = process.stdout.write;
+process.stdout.write = function (chunk, encodingOrCallback, callback) {
+  seen.push(`stdout declared3 ${arguments.length} ${typeof callback}`);
+  return true;
+};
+process.stdout.write("one");
+process.stdout.write("two", "utf8");
+process.stdout.write("three", "utf8", () => {});
+process.stdout.write = function () {
+  seen.push(`stdout declared0 ${arguments.length} ${arguments[0]}`);
+  return true;
+};
+process.stdout.write("zero");
+process.stdout.write = function (chunk) {
+  seen.push(`stdout declared1 ${arguments.length} ${arguments[1]}`);
+  return true;
+};
+process.stdout.write("a", "latin1");
+process.stdout.write = originalOut;
+const originalErr = process.stderr.write;
+process.stderr.write = function (chunk, encodingOrCallback, callback) {
+  seen.push(`stderr declared3 ${arguments.length} ${typeof callback}`);
+  return true;
+};
+process.stderr.write("one");
+process.stderr.write("two", "utf8");
+process.stderr.write("three", "utf8", () => {});
+process.stderr.write = function () {
+  seen.push(`stderr declared0 ${arguments.length} ${arguments[0]}`);
+  return true;
+};
+process.stderr.write("zero");
+process.stderr.write = function (chunk) {
+  seen.push(`stderr declared1 ${arguments.length} ${arguments[1]}`);
+  return true;
+};
+process.stderr.write("a", "latin1");
+process.stderr.write = originalErr;
+console.log(seen.join("\n"));
diff --git a/tests/harness/corpus-costs/stdio-write-replacement-arguments.cjs.json b/tests/harness/corpus-costs/stdio-write-replacement-arguments.cjs.json
new file mode 100644
index 000000000..2b44f308c
--- /dev/null
+++ b/tests/harness/corpus-costs/stdio-write-replacement-arguments.cjs.json
@@ -0,0 +1 @@
+{ "plain": 3757, "san": 5000 }