[libcxx-commits] [libcxx] [libc++] Add the __is_replaceable type trait (PR #132408)

[Louis Dionne via libcxx-commits]

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@@ -65,8 +65,11 @@ class _LIBCPP_EXPORTED_FROM_ABI exception_ptr {
   friend _LIBCPP_HIDE_FROM_ABI exception_ptr make_exception_ptr(_Ep) _NOEXCEPT;
 
 public:
-  // exception_ptr is basically a COW string.
+  // exception_ptr is basically a COW string so it is trivially relocatable.
+  // However, it's not replaceable because destroying and move-constructing could cause
+  // the underlying refcount to hit 0 if we're self-assigning.
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ldionne wrote:

That makes me wonder how useful `__is_replaceable` is as a trait outside of relocation use cases. It might not be.

In order to make progress for the time being, I will add documentation clarifying that `__is_replaceable` doesn't apply if you're doing a self-assignment. And then I will revisit whether these types can be replaceable or not based on that new point of view.

https://github.com/llvm/llvm-project/pull/132408