[clang] [C++20][Modules] Add a test for field info assertion failure. (PR #155948)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Michael Park (mpark)

<details>
<summary>Changes</summary>

I'm having a hard time figuring out what the expected behavior is for this case.

If I get rid of the anonymous union and just keep `S` like this:
```
template &lt;typename T&gt;
struct S { T x; };
```
There are no issues. However, I see that merging occurs to avoid any issues in 
this case. With the `union`, merging logic also occurs there but it doesn't 
seem to work out properly. But, I'm not sure if this should be triggering 
merging logic at all?

If I structure the `main.cpp` like this:
```
import "hu-01.h";
import "hu-02.h";

void g(S&lt;int&gt;) {}
void h() { f(); }
```
There are also no issues, and this time there doesn't seem to be any merging 
either.

**Question**: Should this case (1) not be merging at all? or (2) is it correct 
to be merging, and we need to fix the logic for merging anonymous `union`s?

---
Full diff: https://github.com/llvm/llvm-project/pull/155948.diff


1 Files Affected:

- (added) clang/test/Modules/anon-union-in-template.cpp (+47) 


``````````diff
diff --git a/clang/test/Modules/anon-union-in-template.cpp 
b/clang/test/Modules/anon-union-in-template.cpp
new file mode 100644
index 0000000000000..97fcdc7db86be
--- /dev/null
+++ b/clang/test/Modules/anon-union-in-template.cpp
@@ -0,0 +1,47 @@
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: split-file %s %t
+
+// RUN: %clang_cc1 -std=c++20 -fmodule-name=hu-01 -emit-header-unit 
-xc++-user-header %t/hu-01.h \
+// RUN:  -o %t/hu-01.pcm
+
+// RUN: %clang_cc1 -std=c++20 -fmodule-name=hu-02 -emit-header-unit 
-xc++-user-header %t/hu-02.h \
+// RUN:  -Wno-experimental-header-units \
+// RUN:  -fmodule-map-file=%t/hu-01.map -fmodule-file=hu-01=%t/hu-01.pcm \
+// RUN:  -o %t/hu-02.pcm
+
+// RUN: %clang_cc1 -std=c++20 -emit-obj %t/main.cpp \
+// RUN:  -Wno-experimental-header-units \
+// RUN:  -fmodule-map-file=%t/hu-01.map -fmodule-file=hu-01=%t/hu-01.pcm \
+// RUN:  -fmodule-map-file=%t/hu-02.map -fmodule-file=hu-02=%t/hu-02.pcm
+
+//--- hu-01.map
+module "hu-01" {
+  header "hu-01.h"
+  export *
+}
+
+//--- hu-02.map
+module "hu-02" {
+  header "hu-02.h"
+  export *
+}
+
+//--- hu-01.h
+#pragma once
+
+template <typename T>
+struct S { union { T x; }; };
+
+using SI = S<int>;
+
+//--- hu-02.h
+import "hu-01.h";
+inline void f(S<int> s = {}) { s.x; }
+
+//--- main.cpp
+import "hu-01.h";
+void g(S<int>) {}
+
+import "hu-02.h";
+void h() { f(); }

``````````

</details>


https://github.com/llvm/llvm-project/pull/155948
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