[clang] [clang] add test verifying DR242 behavior (PR #201441)

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<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Jess Ding (jessding)

<details>
<summary>Changes</summary>

Adding a test to document CD4 resolution of [DR 242](https://cplusplus.github.io/CWG/issues/242.html) (currently availability in clang is [marked as unknown](https://clang.llvm.org/cxx_dr_status.html)).

I used Claude Code to understand more about clang/the tests/the repo, but wrote the test code myself.

[description WIP, need to add in responses to the below. will un-draft this PR after I've gone through the policies.]


---
Full diff: https://github.com/llvm/llvm-project/pull/201441.diff


1 Files Affected:

- (modified) clang/test/CXX/drs/cwg2xx.cpp (+31) 


``````````diff
diff --git a/clang/test/CXX/drs/cwg2xx.cpp b/clang/test/CXX/drs/cwg2xx.cpp
index f81ab02e7d748..2ca9a84d0a03a 100644
--- a/clang/test/CXX/drs/cwg2xx.cpp
+++ b/clang/test/CXX/drs/cwg2xx.cpp
@@ -695,6 +695,37 @@ namespace cwg241 { // cwg241: 9
   }
 } // namespace cwg241
 
+namespace cwg242 { // cwg242: yes
+  struct A {};
+  struct I1 : A {};
+  struct I2 : A {};
+  struct D : I1, I2 {};
+
+  A *upcast(D *p) {
+    return (A *)(p);
+    /* expected-error at -1
+    {{ambiguous conversion from derived class 'D' to base class 'A':
+    struct cwg242::D -> I1 -> A
+    struct cwg242::D -> I2 -> A}}*/
+  }
+
+  D *downcast(A *p) {
+    return (D *)(p);
+    /* expected-error at -1
+    {{ambiguous cast from base 'cwg242::A' to derived 'cwg242::D':
+    A -> I1 -> struct cwg242::D
+    A -> I2 -> struct cwg242::D}}*/
+  }
+
+  struct V {};
+  struct B : virtual V {};
+
+  B *virt_downcast(V *p) {
+    return (B *)(p);
+    // expected-error at -1 {{cannot cast 'cwg242::V *' to 'B *' via virtual base 'cwg242::V'}}
+  }
+} // namespace cwg242
+
 namespace cwg243 { // cwg243: 2.8
   struct B;
   struct A {

``````````

</details>


https://github.com/llvm/llvm-project/pull/201441