[Modules] Add test for merging of template member parent

[Hahnfeld]

Summary

This is a reduced test case originally meant to be addressed by https://reviews.llvm.org/D137787. It was recently fixed by commit rG61c7a9140b ("Commit to a primary definition for a class when we load its first member."), noting the difficulty to come up with a reduced test case. This setup with four modules seems to fail consistently before the fix mentioned above.

[rsmith]

Awesome, thanks! It makes perfect sense that rG61c7a9140b would fix this.

[ChuanqiXu]

LGTM. Thanks.

clang/test/Modules/merge-template-members-parent.cpp

+// RUN: rm -rf %t
+// RUN: mkdir %t
+// RUN: split-file %s %t
+
+// RUN: %clang_cc1 -emit-obj -fmodules -fimplicit-module-maps -fmodules-cache-path=%t %t/merge.cpp -o %t/merge.o
+
+//--- V.h
+#ifndef V_H
+#define V_H
+template <typename T>
+struct V {
+  ~V() {}
+};
+#endif
+
+//--- A.h
+#include "V.h"
+
+void A(const V<unsigned long> &v);
+
+//--- B.h
+#include "V.h"
+
+inline V<unsigned long> B() {
+  return {};
+}
+
+//--- C.h
+#include "V.h"
+
+#include "A.h"
+
+class C {
+public:
+  C(const V<unsigned long> &v) {
+    V<unsigned long> v2;
+  }
+};
+
+C GetC() {
+   return {{}};
+}
+
+// This include *MUST* come last.
+#include "B.h"
+
+//--- module.modulemap
+module "V" { header "V.h" export * }
+module "A" { header "A.h" export * }
+module "B" { header "B.h" export * }
+module "C" { header "C.h" export * }
+
+//--- merge.cpp
+#include "C.h"
+
+template <typename T>
+C GetC_main() {
+   return {{}};
+}
+
+void f() {
+   GetC_main<float>();
+   GetC();
+}