[LLVMbugs] [Bug 22426] generic lambdas, decltype(auto), and rvalue references, oh my!

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=22426

Richard Smith <richard-llvm at metafoo.co.uk> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
                 CC|                            |richard-llvm at metafoo.co.uk
         Resolution|---                         |INVALID

--- Comment #2 from Richard Smith <richard-llvm at metafoo.co.uk> ---
(In reply to comment #0)
> There should *never* be an error when using decltype(auto).

That is true if the returned expression is a function call, but there are a few
cases where decltype(auto) returns can fail[1]. This is one of them.

decltype applied to the returned expression returns the declared type of the
member, due to a special case for class member access. In this case, the
declared type of the member is 'int &&', but the member access expression is an
int lvalue, so the reference binding fails.

You can get the result you expect here by a syntactic contortion: add another
pair of parens around your returned expression:

  return (((decltype(p)&&)p).first);

This causes decltype to ignore that it's in the 'class member access
expression' special case. Note that this special case is in fact essential in
other uses of decltype(auto):

  struct Q { int n; };
  decltype(auto) f(Q q) { return q.n; }

Without the special case that is causing you problems here, the above function
would have return type 'int &' rather than 'int', which is obviously bad.



 [1] This can happen in two ways:
    1) the computed type is *not* the type of the expression plus & or && for
an lvalue or xvalue (this happens if we fall into [dcl.type.simple]/4.1)
    2) the expression can't actually be used to initialize an object of its own
type

We're in case (1) here; for completeness, an example of case (2) would be:

  struct X { int n : 1; };
  decltype(auto) f(X x) { return (x.n); }

Here, decltype(auto) computes the type int&, which cannot bind to a bit-field.
Here's another:

  unique_ptr<int> u;
  decltype(auto) f() { return u; }

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