[libcxx-commits] [clang] [libcxx] [clang] Reland: Instantiate concepts with sugared template arguments (PR #101782)

[Zequan Wu via libcxx-commits]

ZequanWu wrote:

This change makes the following code no longer compile:
```
#include <concepts>
#include <iterator>

template <typename T, typename It>
concept CompatibleIter = std::contiguous_iterator<It>;

template <typename T>
class span {
  public:
  template <typename It>
    requires(CompatibleIter<T, It>)
  constexpr span(It first, int count) noexcept {

  }
};

void foo(int n) {
  int vla[n];
  span<const int> s(vla, n);
}
```
```
$ clang++  span.cpp -c -std=c++20
span.cpp:18:11: warning: variable length arrays in C++ are a Clang extension [-Wvla-cxx-extension]
   18 |   int vla[n];
      |           ^
span.cpp:18:11: note: function parameter 'n' with unknown value cannot be used in a constant expression
span.cpp:17:14: note: declared here
   17 | void foo(int n) {
      |              ^
span.cpp:19:19: error: no matching constructor for initialization of 'span<const int>'
   19 |   span<const int> s(vla, n);
      |                   ^ ~~~~~~
span.cpp:12:13: note: candidate template ignored: constraints not satisfied [with It = int *]
   12 |   constexpr span(It first, int count) noexcept {
      |             ^
span.cpp:11:14: note: because 'CompatibleIter<const int, int *>' evaluated to false
   11 |     requires(CompatibleIter<T, It>)
      |              ^
span.cpp:5:26: note: because 'int *' does not satisfy 'contiguous_iterator'
    5 | concept CompatibleIter = std::contiguous_iterator<It>;
      |                          ^
/usr/lib/gcc/x86_64-linux-gnu/14/../../../../include/c++/14/bits/iterator_concepts.h:697:35: note: because 'int *' does not satisfy 'random_access_iterator'
  697 |     concept contiguous_iterator = random_access_iterator<_Iter>
      |                                   ^
/usr/lib/gcc/x86_64-linux-gnu/14/../../../../include/c++/14/bits/iterator_concepts.h:681:38: note: because 'int *' does not satisfy 'bidirectional_iterator'
  681 |     concept random_access_iterator = bidirectional_iterator<_Iter>
      |                                      ^
/usr/lib/gcc/x86_64-linux-gnu/14/../../../../include/c++/14/bits/iterator_concepts.h:671:38: note: because 'int *' does not satisfy 'forward_iterator'
  671 |     concept bidirectional_iterator = forward_iterator<_Iter>
      |                                      ^
/usr/lib/gcc/x86_64-linux-gnu/14/../../../../include/c++/14/bits/iterator_concepts.h:666:32: note: because 'int *' does not satisfy 'input_iterator'
  666 |     concept forward_iterator = input_iterator<_Iter>
      |                                ^
/usr/lib/gcc/x86_64-linux-gnu/14/../../../../include/c++/14/bits/iterator_concepts.h:655:30: note: because 'int *' does not satisfy 'input_or_output_iterator'
  655 |     concept input_iterator = input_or_output_iterator<_Iter>
      |                              ^
/usr/lib/gcc/x86_64-linux-gnu/14/../../../../include/c++/14/bits/iterator_concepts.h:635:5: note: because 'int *' does not satisfy 'weakly_incrementable'
  635 |         && weakly_incrementable<_Iter>;
      |            ^
/usr/lib/gcc/x86_64-linux-gnu/14/../../../../include/c++/14/bits/iterator_concepts.h:619:36: note: because 'int *' does not satisfy 'movable'
  619 |     concept weakly_incrementable = movable<_Iter>
      |                                    ^
/usr/lib/gcc/x86_64-linux-gnu/14/../../../../include/c++/14/concepts:265:10: note: because 'assignable_from<int *&, int *>' evaluated to false
  265 |       && assignable_from<_Tp&, _Tp> && swappable<_Tp>;
      |          ^
/usr/lib/gcc/x86_64-linux-gnu/14/../../../../include/c++/14/concepts:143:44: note: because 'same_as<expr-type, int *&>' would be invalid
  143 |         { __lhs = static_cast<_Rhs&&>(__rhs) } -> same_as<_Lhs>;
      |                                                   ^
span.cpp:8:7: note: candidate constructor (the implicit copy constructor) not viable: requires 1 argument, but 2 were provided
    8 | class span {
      |       ^~~~
span.cpp:8:7: note: candidate constructor (the implicit move constructor) not viable: requires 1 argument, but 2 were provided
    8 | class span {
      |       ^~~~
1 warning and 1 error generated.
```

Can you revert it if it takes a while to fix?

https://github.com/llvm/llvm-project/pull/101782