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std::vector and other containers (except std::array) are specified to have a copy constructor. This is not specified to be conditional on whether or not the element type is copyable. Only instantiation of the copy constructor's definition is forbidden if the element type is not copyable.

As a result std::is_copy_constructible_v on the container will always be true. There is no way to test whether an instantiation of a definition would be well-formed with a type trait.

It would be possible to specify that the copy constructor is not declared or excluded from overload resolution if the element type is not copyable. However, that would come with a trade-off which is explained in detail in this blog post: https://quuxplusone.github.io/blog/2020/02/05/vector-is-copyable-except-when-its-not/.

In short, if we want to be able to use the container with an incomplete type, e.g. recursively like

As stated in the comments, g is odr-used. However, there is a definition for it available, so there is no non-diagnosable violation here; MSVC is wrong to accept it. (This is true even without the constructor declaration; the implicitly declared B::B() is never defined, but the default member initializer is still an odr-use like it is here.)

In addition to the provided comments:

%$% also called the Exposition pipe vs. %>%:

This is a short summary of this article https://towardsdatascience.com/3-lesser-known-pipe-operators-in-tidyverse-111d3411803a

"The key difference in using %$% or %>% lies in the type of arguments of used functions."

One advantage, and as far as I can understand it, for me the only one to use %$% over %>% is the fact that we can avoid repetitive input of the dataframe name in functions that have no data as an argument.

For example the lm() has a data argument. In this case we can use both %>% and %$% interchangeable.

But in functions like the cor() which has no data argument:

This is CWG 507. An example similar to yours was given, and the submitter explained that according to the standard, the overload resolution is ambiguous, even though this result is very counter-intuitive.

Translating to your particular example, when comparing operator==(int, int) and operator==(float, int) to determine which is the better candidate, we have to determine which one has the better implicit conversion sequence for the first argument (obviously in the second argument, no conversion is required). For the first argument of operator==(int, int), we just use A::operator int. For the first argument of operator==(float, int), there is no way to decide whether to use A::operator int or A::operator char, so we get the "ambiguous conversion sequence". The overload resolution rules say that the ambiguous conversion sequence is no better or worse than any other user-defined conversion sequence. Therefore, the straightforward conversion from A{} to int (via A::operator int) is not considered better than the ambiguous conversion from A{} to float. This means neither operator== candidate is better than the other.

Clang is apparently following the letter of the standard whereas GCC and MSVC are probably doing something else because of the standard seeming to be broken here. "Which compiler is right" depends on your opinion about what the standard should say. There is no proposed resolution on the issues page.

I would suggest removing operator char unless you really, really need it, in which case you will have to think about what else you're willing to give up.

I think this is the open CWG issue 2077.

Basically,

tic/toc should be fine, but it looks like the timing is being skewed by memory pre-allocation.

I can reproduce similar timings to your MATLAB example, however

• On first run (clear workspace)

  • Loop approach takes 2.08 sec
  • Vectorised approach takes 1.04 sec
  • Vectorisation saves 50% execution time

• On second run (workspace not cleared)

  • Loop approach takes 2.55 sec
  • Vectorised approach takes 0.065 sec
  • Vectorisation "saves" 97.5% execution time

My guess would be that since the loop approach explicitly creates a new matrix via zeros, the memory is reallocated from scratch on every run and you don't see the speed improvement on subsequent runs.

However, when HH remains in memory and the HH=___ line outputs a matrix of the same size, I suspect MATLAB is doing some clever memory allocation to speed up the operation.

We can prove this theory with the following test:

It's all slightly mysterious. gcc behaves the same as clang.

The standard has this to say (emphasis mine):

Absent default member initializers, if any non-static data member of a union has a non-trivial default constructor, copy constructor, move constructor, copy assignment operator, move assignment operator, or destructor, the corresponding member function of the union must be user-provided or it will be implicitly deleted for the union.

But I think the wording is a bit wooly here. I think what they actually mean is that you must provide an initialiser for the member that has (in your example) a non-trivial constructor, like so:

You can turn Duration(int t_seconds) into a template function that can accept an int and set it to deprecated.

When RFC 3013, "Checking conditional compilation at compile time", is implemented, there will be warnings for a #[cfg] referring to a feature name that is not declared by Cargo, just as you're asking for. However, the implementation only just got started (Sep 28, 2021).

The means of operation described in the RFC is just as you suggested, ‘cargo would pass down the flags to rustc’.

It may be worth noting that this will not check all conditions appearing in the source text; as described in the RFC:

This lint will not be able to detect invalid #[cfg] tests that are within modules that are not compiled, presumably because an ancestor mod is disabled.

So, it will not confirm that all #[cfg(feature)] are valid on a single cargo check — you will need to test with your various features or combinations of features. But those are the same combinations that you would need anyway to check for compile errors in all of the regular source code that could be enabled; once you do that, the lint will assure you that you don't have any #[cfg(feature)] that are never enabled due to a typo.