[llvm-bugs] [Bug 39728] New: Trivial type with defaulted assignment and destructor is not usable in constexpr

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=39728

            Bug ID: 39728
           Summary: Trivial type with defaulted assignment and destructor
                    is not usable in constexpr
           Product: clang
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Frontend
          Assignee: unassignedclangbugs at nondot.org
          Reporter: ldionne at apple.com
                CC: llvm-bugs at lists.llvm.org, neeilans at live.com,
                    richard-llvm at metafoo.co.uk

The following code does not compile under Clang trunk (GCC compiles it):

----------------------------------------------------------
#include <type_traits>

struct Trivial {
    Trivial& operator=(Trivial const&) = default;
    ~Trivial() = default;
};

static_assert(std::is_trivially_copy_assignable<Trivial>::value);
static_assert(std::is_trivially_destructible<Trivial>::value);

constexpr bool use() { 
  Trivial a{}, b{};
  a = b;
  return true;
}

static_assert(use());
----------------------------------------------------------

The command I use to reproduce is this:

----------------------------------------------------------
$ pbpaste | clang++ -std=c++2a -xc++ -fsyntax-only -
<stdin>:11:16: error: constexpr function never produces a constant expression
[-Winvalid-constexpr]
constexpr bool use() {
               ^
<stdin>:13:5: note: non-constexpr function 'operator=' cannot be used in a
constant expression
  a = b;
    ^
<stdin>:4:14: note: declared here
    Trivial& operator=(Trivial const&) = default;
             ^
<stdin>:17:15: error: static_assert expression is not an integral constant
expression
static_assert(use());
              ^~~~~
<stdin>:13:5: note: non-constexpr function 'operator=' cannot be used in a
constant expression
  a = b;
    ^
<stdin>:17:15: note: in call to 'use()'
static_assert(use());
              ^
<stdin>:4:14: note: declared here
    Trivial& operator=(Trivial const&) = default;
             ^
2 errors generated.
----------------------------------------------------------

Live example with Clang trunk: https://wandbox.org/permlink/gJWRWbzxfduLQnc5
Live example with GCC trunk: https://wandbox.org/permlink/jJU6pW9S6rAYdGrH

The problem seems to be that the defaulted assignment operator is not
constexpr. I think it should be (especially since it's trivial).

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