[llvm-bugs] [Bug 43086] New: constexpr variable initialisation error after forwarding reference

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=43086

            Bug ID: 43086
           Summary: constexpr variable initialisation error after
                    forwarding reference
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++11
          Assignee: unassignedclangbugs at nondot.org
          Reporter: pkeir at outlook.com
                CC: blitzrakete at gmail.com, dgregor at apple.com,
                    erik.pilkington at gmail.com, llvm-bugs at lists.llvm.org,
                    richard-llvm at metafoo.co.uk

Created attachment 22413
  --> https://bugs.llvm.org/attachment.cgi?id=22413&action=edit
Code as described.

The C++11 code below (also attached) will fail to compile:

struct Bar {};
struct Foo { Bar t; };

template <typename T>
constexpr T jam(T &&a) { return a; }

void test()
{
  Foo y;
  constexpr Foo x = jam(y);
}

Changing the types of x and y from Foo to Bar will remove the error. GCC
snapshot 17th August 2019 produces no error. The error message follows:

constexpr_rvalue.cpp:10:17: error: constexpr variable 'x' must be initialized
by a constant expression
  constexpr Foo x = jam(y);
                ^   ~~~~~~
constexpr_rvalue.cpp:10:21: note: read of non-constexpr variable 'y' is not
allowed in a constant expression
  constexpr Foo x = jam(y);
                    ^
constexpr_rvalue.cpp:10:21: note: in call to 'Foo(y)'
constexpr_rvalue.cpp:9:7: note: declared here
  Foo y;
      ^
1 error generated.

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