[modules] Fix assert on Clang module import from the global module fragment. (#159771)

Fixes #159768.

When building a named module interface with `-fmodules` enabled,
importing a Clang module from inside the global module fragment causes
Clang to crash only on assertion builds.
This fixes the assert and extends the test coverage.

1 files changed, 6 insertions, 1 deletions

diff --git a/clang/lib/Sema/SemaModule.cpp b/clang/lib/Sema/SemaModule.cpp
index 773bcb2..a2aa3ea 100644
--- a/clang/lib/Sema/SemaModule.cpp
+++ b/clang/lib/Sema/SemaModule.cpp
@@ -772,7 +772,12 @@ void Sema::BuildModuleInclude(SourceLocation DirectiveLoc, Module *Mod) {
     Module *ThisModule = PP.getHeaderSearchInfo().lookupModule(
         getLangOpts().CurrentModule, DirectiveLoc, false, false);
     (void)ThisModule;
-    assert(ThisModule && "was expecting a module if building one");
+    // For named modules, the current module name is not known while parsing the
+    // global module fragment and lookupModule may return null.
+    assert((getLangOpts().getCompilingModule() ==
+                LangOptionsBase::CMK_ModuleInterface ||
+            ThisModule) &&
+           "was expecting a module if building a Clang module");
   }
 }