[libcxx-commits] [libcxx] [libcxx] improves diagnostics for containers with bad value types (PR #106296)

[Christopher Di Bella via libcxx-commits]

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@@ -76,8 +79,16 @@ struct __non_trivial_if<true, _Unique> {
 
 template <class _Tp>
 class _LIBCPP_TEMPLATE_VIS allocator : private __non_trivial_if<!is_void<_Tp>::value, allocator<_Tp> > {
-  static_assert(!is_const<_Tp>::value, "std::allocator does not support const types");
-  static_assert(!is_volatile<_Tp>::value, "std::allocator does not support volatile types");
+  static_assert(!is_const<_Tp>::value, "'std::allocator' can only allocate non-const object types");
+  static_assert(!is_volatile<_Tp>::value, "'std::allocator' can only allocate non-volatile object types");
+  static_assert(!is_reference<_Tp>::value || !is_function<typename remove_reference<_Tp>::type>::value,
+                "'std::allocator' can only allocate object types; function references are not objects (consider using "
+                "a function pointer)");
+  static_assert(!is_reference<_Tp>::value,
+                "'std::allocator' can only allocate object types; references are not objects");
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cjdb wrote:

I kicked the internal discussion off by asking if the average C++ programmer would be able to understand ` 'std::vector' can only allocate non-cv-qualified object types`, and the feedback I've received can be aggregated as roughly

* most C++ programmers don't know what the term "cv-qualified" means
* we should only mention relevant information, and mentioning "volatile" when `volatile` isn't used was repeatedly called out in particular
* most C++ programmers probably aren't going to know that "object" has a very different meaning to other languages
* most non-toolchain folks in the discussion seemed to prefer separate messages; toolchain folks were sort of ambivalent

What do you feel a single check will offer over independent checks?

https://github.com/llvm/llvm-project/pull/106296