When making modules transitively visible, don't take into account

whether they have missing header files.

Whether a module's headers happen to be present on the local file system
should make no difference to whether we make its contents visible when
importing another module that re-exports it. If we have an up-to-date
AST file that we can load, that's all that matters.

This fixes the ability to header syntax checking for modular headers in
C++20 mode (or in prior modes where -fmodules-local-submodule-visibility
is enabled but -fmodules is not).

1 files changed, 3 insertions, 0 deletions

diff --git a/clang/lib/Lex/ModuleMap.cpp b/clang/lib/Lex/ModuleMap.cpp
index 2c79bb2..4f7d5ab 100644
--- a/clang/lib/Lex/ModuleMap.cpp
+++ b/clang/lib/Lex/ModuleMap.cpp
@@ -544,6 +544,9 @@ void ModuleMap::diagnoseHeaderInclusion(Module *RequestingModule,
 static bool isBetterKnownHeader(const ModuleMap::KnownHeader &New,
                                 const ModuleMap::KnownHeader &Old) {
   // Prefer available modules.
+  // FIXME: Considering whether the module is available rather than merely
+  // importable is non-hermetic and can result in surprising behavior for
+  // prebuilt modules. Consider only checking for importability here.
   if (New.getModule()->isAvailable() && !Old.getModule()->isAvailable())
     return true;