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

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

[Marek Polacek]

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.
---
 gcc/cp/pt.c                                 | 2 ++
 gcc/testsuite/g++.dg/cpp1y/decltype-auto1.C | 8 ++++++++
 2 files changed, 10 insertions(+)
 create mode 100644 gcc/testsuite/g++.dg/cpp1y/decltype-auto1.C

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index f401c75b9e5..c033a286407 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -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)
diff --git a/gcc/testsuite/g++.dg/cpp1y/decltype-auto1.C b/gcc/testsuite/g++.dg/cpp1y/decltype-auto1.C
new file mode 100644
index 00000000000..13baf8eba06
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/decltype-auto1.C
@@ -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" }
+}

base-commit: d16d45655d77d58e3f8430b9cf386b04759e01c7
-- 
2.28.0

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

[Jason Merrill]

On 11/5/20 3:52 PM, Marek Polacek wrote:
> 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.

OK.

> 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.
> ---
>   gcc/cp/pt.c                                 | 2 ++
>   gcc/testsuite/g++.dg/cpp1y/decltype-auto1.C | 8 ++++++++
>   2 files changed, 10 insertions(+)
>   create mode 100644 gcc/testsuite/g++.dg/cpp1y/decltype-auto1.C
> 
> diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
> index f401c75b9e5..c033a286407 100644
> --- a/gcc/cp/pt.c
> +++ b/gcc/cp/pt.c
> @@ -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)
> diff --git a/gcc/testsuite/g++.dg/cpp1y/decltype-auto1.C b/gcc/testsuite/g++.dg/cpp1y/decltype-auto1.C
> new file mode 100644
> index 00000000000..13baf8eba06
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp1y/decltype-auto1.C
> @@ -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" }
> +}
> 
> base-commit: d16d45655d77d58e3f8430b9cf386b04759e01c7
>