[clang] [Clang] Clarify the `[[trivial_abi]]` documentation. (PR #143243)

[Erich Keane via cfe-commits]

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@@ -4431,7 +4431,11 @@ destroy the object before returning. The lifetime of the copy of the parameter
 in the caller ends without a destructor call when the call begins.
 
 If a type is trivial for the purpose of calls, it is assumed to be trivially
-relocatable for the purpose of ``__is_trivially_relocatable``.
+relocatable for the purpose of ``__is_trivially_relocatable`` and
+``__builtin_is_cpp_trivially_relocatable``.
+The copy constructor of an object of such type might not be called
+when the object is passed to a function. Therefore, the ``trivial_abi``
+attribute should not be applied to objects that contain pointers to themselves.
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erichkeane wrote:

I might suggest generalizing this more... it isn't just 'pointer to themselves', it is 'does something meaningful in the copy constructor' or the 'copy constructor has some meaningful side effects'.  <dont take either fo those, neither are right, but hopefully a jumping off point).



https://github.com/llvm/llvm-project/pull/143243