[clang][modules] Ensure -nostdlib causes no manifest to be reported

[naveen-seth]

When -nostdlib is specified, Clang should not report any library‑provided module manifest, even if a manifest for the default standard library is present.

[llvmbot]

@llvm/pr-subscribers-clang-driver
@llvm/pr-subscribers-clang-modules

@llvm/pr-subscribers-clang

Author: Naveen Seth Hanig (naveen-seth)

Changes

When -nostdlib is specified, Clang should not report any library‑provided module manifest, even if a manifest for the default standard library is present.

---

Full diff: https://github.com/llvm/llvm-project/pull/161110.diff

2 Files Affected:

• (modified) clang/lib/Driver/Driver.cpp (+3)
• (modified) clang/test/Driver/modules-print-library-module-manifest-path.cpp (+8)

diff --git a/clang/lib/Driver/Driver.cpp b/clang/lib/Driver/Driver.cpp
index f110dbab3e5a5..85a1335785542 100644
--- a/clang/lib/Driver/Driver.cpp
+++ b/clang/lib/Driver/Driver.cpp
@@ -6613,6 +6613,9 @@ std::string Driver::GetStdModuleManifestPath(const Compilation &C,
                                              const ToolChain &TC) const {
   std::string error = "<NOT PRESENT>";
 
+  if (C.getArgs().hasArg(options::OPT_nostdlib))
+    return error;
+
   switch (TC.GetCXXStdlibType(C.getArgs())) {
   case ToolChain::CST_Libcxx: {
     auto evaluate = [&](const char *library) -> std::optional<std::string> {
diff --git a/clang/test/Driver/modules-print-library-module-manifest-path.cpp b/clang/test/Driver/modules-print-library-module-manifest-path.cpp
index 7606713bfa22a..ba4c2cd4524cd 100644
--- a/clang/test/Driver/modules-print-library-module-manifest-path.cpp
+++ b/clang/test/Driver/modules-print-library-module-manifest-path.cpp
@@ -18,6 +18,14 @@
 // RUN:     --target=x86_64-linux-gnu 2>&1 \
 // RUN:   | FileCheck libcxx.cpp
 
+// check that -nostdlib causes no library-provided module manifest to
+// be reported, // even when libc++.modules.json is present.
+// RUN: %clang -print-library-module-manifest-path \
+// RUN:     -nostdlib \
+// RUN:     -resource-dir=%t/Inputs/usr/lib/x86_64-linux-gnu \
+// RUN:     --target=x86_64-linux-gnu 2>&1 \
+// RUN:   | FileCheck libcxx-no-module-json.cpp
+
 // for macos there is a different directory structure
 // where the library and libc++.modules.json file are in lib
 // directly but headers are in clang/ver directory which

[naveen-seth]

This ties into #152770. For driver-managed module builds, -nostdlib should ensure that standard library modules are not implicitly imported. The logic doesn’t necessarily have to be here, but I thought it would be a reasonable location.