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c++: Fix decltype(auto) deduction with rvalue ref [PR78209]

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Here's a small deficiency in decltype(auto).  [dcl.type.auto.deduct]/5:
If the placeholder-type-specifier is of the form decltype(auto), [...]
the type deduced for T is determined [...] as though E had been the operand
of the decltype.  So:

  int &&i = 0;
  decltype(auto) j = i; // should behave like int &&j = i; error

We deduce j's type in do_auto_deduction via finish_decltype_type which
takes an 'id' argument.  Currently we compute 'id' as false, because
stripped_init is *i (a REFERENCE_REF_P).  But it seems to me we should
rather set 'id' to true here, by looking through the REFERENCE_REF_P,
so that finish_decltype_type DTRT.

gcc/cp/ChangeLog:

	PR c++/78209
	* pt.c (do_auto_deduction): If init is REFERENCE_REF_P, use its
	first operand.

gcc/testsuite/ChangeLog:

	PR c++/78209
	* g++.dg/cpp1y/decltype-auto1.C: New test.
This commit is contained in:
Marek Polacek 2020-11-03 15:10:31 -05:00
parent 1d87302a8e
commit e6fd02cc6d
2 changed files with 10 additions and 0 deletions
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2
gcc/cp/pt.c
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@ -29278,6 +29278,8 @@ do_auto_deduction (tree type, tree init, tree auto_node,
else if (AUTO_IS_DECLTYPE (auto_node))
{
tree stripped_init = tree_strip_any_location_wrapper (init);
if (REFERENCE_REF_P (stripped_init))
stripped_init = TREE_OPERAND (stripped_init, 0);
bool id = (DECL_P (stripped_init)
|| ((TREE_CODE (init) == COMPONENT_REF
|| TREE_CODE (init) == SCOPE_REF)

8
gcc/testsuite/g++.dg/cpp1y/decltype-auto1.C Normal file
View file

@ -0,0 +1,8 @@
// PR c++/78209
// { dg-do compile { target c++14 } }
int main()
{
int &&i = 0;
decltype(auto) j = i; // { dg-error "cannot bind rvalue reference" }
}