From 0ee069f094d4aa7c880e0b1282250659c4abefc1 Mon Sep 17 00:00:00 2001
From: Chris Tate <chris@ctate.dev>
Date: Sun, 18 Oct 2026 06:53:41 -0500
Subject: [PATCH] Answer typeof for builtin module functions and classes

`typeof zlib.Inflate` and `typeof zlib.createGzip` stopped at SC1090 or
SC2020, because the operand lowered as a library function or class
value. A typeof of a function or class exported by a Node builtin
module, read through a plain binding, now folds to "function" as in
Node: the read has no side effects to preserve.
---
 .../src/frontend/lowering/lower-exprs.ts      | 41 +++++++++++++++++++
 .../5974-typeof-module-exports.cjs.json       |  6 +++
 tests/corpus/5974-typeof-module-exports.cjs   |  9 ++++
 tests/harness/corpus-costs.json               |  2 +
 4 files changed, 58 insertions(+)
 create mode 100644 packages/compiler/test/ts7/baselines/order-parity/tests/corpus/5974-typeof-module-exports.cjs.json
 create mode 100644 tests/corpus/5974-typeof-module-exports.cjs

diff --git a/packages/compiler/src/frontend/lowering/lower-exprs.ts b/packages/compiler/src/frontend/lowering/lower-exprs.ts
index 6774de117..f0538bb71 100644
--- a/packages/compiler/src/frontend/lowering/lower-exprs.ts
+++ b/packages/compiler/src/frontend/lowering/lower-exprs.ts
@@ -648,6 +648,30 @@ function lowerExprInner(lowerer: Lowerer, expr: ts.Expression): IrExpr {
     // have non-stdlib symbols and keep the ordinary path.
     let tested = expr.expression;
     while (ts.isParenthesizedExpression(tested)) tested = tested.expression;
+    // `typeof zlib.Inflate` / `typeof zlib.createGzip`: an export of a
+    // Node builtin module read through a plain binding (an import or
+    // require result). The read has no side effects, and Node's answer for
+    // every exported function and class is "function".
+    if (
+      ts.isPropertyAccessExpression(tested) &&
+      ts.isIdentifier(tested.expression) &&
+      lowerer.isStdlibMember(tested)
+    ) {
+      const sym = lowerer.checker.getSymbolAtLocation(tested.name);
+      const decls = sym ? lowerer.checker.declarationsOf(sym) : [];
+      if (
+        decls.some(isBuiltinModuleValueDecl) &&
+        decls.every((decl) => builtinModuleExportParent(decl) !== undefined)
+      ) {
+        const t = lowerer.typeOf(tested);
+        if (
+          lowerer.checker.getCallSignatures(t).length > 0 ||
+          lowerer.checker.getConstructSignatures(t).length > 0
+        ) {
+          return { kind: "strLit", value: "function", type: STRING, loc };
+        }
+      }
+    }
     if (ts.isIdentifier(tested)) {
       const sym = lowerer.checker.getSymbolAtLocation(tested);
       if (!sym) {
@@ -15825,3 +15849,20 @@ function checkedIteratorPack(lowerer: Lowerer, source: IrExpr, loc: SrcLoc): IrE
   }
   return { kind: "call", callee: name, args: [source], type: DYN, loc };
 }
+
+/** The `declare module "..."` body (or nested namespace body) a module
+ * export's declaration sits directly in, or undefined. */
+function builtinModuleExportParent(decl: ts.Node): ts.ModuleBlock | undefined {
+  const parent = ts.isVariableDeclaration(decl) ? decl.parent?.parent?.parent : decl.parent;
+  return parent !== undefined && ts.isModuleBlock(parent) ? parent : undefined;
+}
+
+/** A function, class or variable exported by a builtin module's body. */
+function isBuiltinModuleValueDecl(decl: ts.Node): boolean {
+  return (
+    builtinModuleExportParent(decl) !== undefined &&
+    (ts.isFunctionDeclaration(decl) ||
+      ts.isClassDeclaration(decl) ||
+      ts.isVariableDeclaration(decl))
+  );
+}
diff --git a/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/5974-typeof-module-exports.cjs.json b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/5974-typeof-module-exports.cjs.json
new file mode 100644
index 000000000..469ee3272
--- /dev/null
+++ b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/5974-typeof-module-exports.cjs.json
@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/5974-typeof-module-exports.cjs"
+ ],
+ "diags": []
+}
diff --git a/tests/corpus/5974-typeof-module-exports.cjs b/tests/corpus/5974-typeof-module-exports.cjs
new file mode 100644
index 000000000..7e209f147
--- /dev/null
+++ b/tests/corpus/5974-typeof-module-exports.cjs
@@ -0,0 +1,9 @@
+// typeof on functions and classes exported by Node builtin modules, read
+// through require bindings: Node answers "function" for each.
+const zlib = require("node:zlib");
+const path = require("node:path");
+const fs = require("node:fs");
+console.log(typeof zlib.createGzip, typeof zlib.createBrotliDecompress, typeof zlib.gunzipSync);
+console.log(typeof zlib.Inflate, typeof zlib.Gzip, typeof zlib.DeflateRaw);
+console.log(typeof path.join, typeof path.posix.join, typeof fs.readFileSync);
+console.log(typeof path.sep);
diff --git a/tests/harness/corpus-costs.json b/tests/harness/corpus-costs.json
index 2c2eeab31..25f00b1f7 100644
--- a/tests/harness/corpus-costs.json
+++ b/tests/harness/corpus-costs.json
@@ -1908,6 +1908,7 @@
     "5970-net-connect-host-name.ts": 2400,
     "5971-buffer-chunks-to-string.ts": 2700,
     "5972-net-set-encoding-listeners.ts": 2550,
+    "5974-typeof-module-exports.cjs": 2000,
     "5990-worker-uncloneable.mjs": 3283,
     "5991-worker-async-dispose.mjs": 5699,
     "5992-worker-thread-name-lifetime.mjs": 2687,
@@ -4603,6 +4604,7 @@
     "5970-net-connect-host-name.ts": 2800,
     "5971-buffer-chunks-to-string.ts": 3100,
     "5972-net-set-encoding-listeners.ts": 3000,
+    "5974-typeof-module-exports.cjs": 2500,
     "5990-worker-uncloneable.mjs": 22432,
     "5991-worker-async-dispose.mjs": 25768,
     "5992-worker-thread-name-lifetime.mjs": 28932,