Add a unit test that reproduces the assertion failure during bootstrap

build on OS X with XCode.

Author: mpark

clang/test/Modules/pr129982.cpp

@@ -0,0 +1,107 @@
+// If this test fails, it should be investigated under Debug builds.
+// Before the PR, this test was violating an assertion.
+
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: split-file %s %t
+
+// RUN: %clang_cc1 -std=c++20 -emit-obj -fmodules \
+// RUN:  -fmodule-map-file=%t/module.modulemap \
+// RUN:  -fmodules-cache-path=%t %t/a.cpp
+
+//--- module.modulemap
+module ebo {
+  header "ebo.h"
+}
+
+module fwd {
+  header "fwd.h"
+}
+
+module s {
+  header "s.h"
+  export *
+}
+
+module mod {
+  header "a.h"
+  header "b.h"
+}
+
+//--- ebo.h
+#pragma once
+
+namespace N { inline namespace __1 {
+
+template <typename T>
+struct EBO : T {
+  EBO() = default;
+};
+
+}}
+
+//--- fwd.h
+#pragma once
+
+namespace N { inline namespace __1 {
+
+template <typename T>
+struct Empty;
+
+template <typename T>
+struct BS;
+
+using S = BS<Empty<char>>;
+
+}}
+
+//--- s.h
+#pragma once
+
+#include "fwd.h"
+#include "ebo.h"
+
+namespace N { inline namespace __1 {
+
+template <typename T>
+struct Empty {};
+
+template <typename T>
+struct BS {
+    EBO<T> _;
+    void f();
+};
+
+extern template void BS<Empty<char>>::f();
+
+}}
+
+//--- b.h
+#pragma once
+
+#include "s.h"
+
+struct B {
+  void f() {
+    N::S{}.f();
+  }
+};
+
+//--- a.h
+#pragma once
+
+#include "s.h"
+
+struct A {
+  void f(int) {}
+  void f(const N::S &) {}
+
+  void g();
+};
+
+//--- a.cpp
+#include "a.h"
+
+void A::g() { f(0); }
+
+// expected-no-diagnostics