c++: Fix for decltype(auto) and parenthesized expr [PR103403]

  In r11-4758, I tried to fix this problem:

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

wherein do_auto_deduction was getting confused with a REFERENCE_REF_P
and it didn't realize its operand was a name, not an expression, and
deduced the wrong type.

Unfortunately that fix broke this:

  int&& r = 1;
  decltype(auto) rr = (r);

where 'rr' should be 'int &' since '(r)' is an expression, not a name.  But
because I stripped the INDIRECT_REF with the r11-4758 change, we deduced
'rr's type as if decltype had gotten a name, resulting in 'int &&'.

I suspect I thought that the REF_PARENTHESIZED_P check when setting
'bool id' in do_auto_deduction would handle the (r) case, but that's not
the case; while the documentation for REF_PARENTHESIZED_P specifically says
it can be set in INDIRECT_REF, we don't actually do so.

This patch sets REF_PARENTHESIZED_P even on REFERENCE_REF_P, so that
do_auto_deduction can use it.

It also removes code in maybe_undo_parenthesized_ref that I think is
dead -- and we don't hit it while running dg.exp.  To adduce more data,
it also looks dead here:
https://splichal.eu/lcov/gcc/cp/semantics.c.gcov.html

Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk and 11?

	PR c++/103403

gcc/cp/ChangeLog:

	* cp-gimplify.c (cp_fold): Don't recurse if maybe_undo_parenthesized_ref
	doesn't change its argument.
	* pt.c (do_auto_deduction): Don't strip REFERENCE_REF_P trees.  Also
	REF_PARENTHESIZED_P for REFERENCE_REF_P.
	* semantics.c (force_paren_expr): Set REF_PARENTHESIZED_P on
	REFERENCE_REF_P trees too.
	(maybe_undo_parenthesized_ref): Remove dead code.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/decltype-auto2.C: New test.
	* g++.dg/cpp1y/decltype-auto3.C: New test.
---
 gcc/cp/cp-gimplify.c                        |  3 ++-
 gcc/cp/pt.c                                 |  5 ++---
 gcc/cp/semantics.c                          | 18 ++++--------------
 gcc/testsuite/g++.dg/cpp1y/decltype-auto2.C | 12 ++++++++++++
 gcc/testsuite/g++.dg/cpp1y/decltype-auto3.C | 12 ++++++++++++
 5 files changed, 32 insertions(+), 18 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp1y/decltype-auto2.C
 create mode 100644 gcc/testsuite/g++.dg/cpp1y/decltype-auto3.C


base-commit: 1e625a44f6f3001cea31e0f7c563943ecba92b68
  

On 12/1/21 10:16, Marek Polacek wrote:
> In r11-4758, I tried to fix this problem:
> 
>    int &&i = 0;
>    decltype(auto) j = i; // should behave like int &&j = i; error
> 
> wherein do_auto_deduction was getting confused with a REFERENCE_REF_P
> and it didn't realize its operand was a name, not an expression, and
> deduced the wrong type.
> 
> Unfortunately that fix broke this:
> 
>    int&& r = 1;
>    decltype(auto) rr = (r);
> 
> where 'rr' should be 'int &' since '(r)' is an expression, not a name.  But
> because I stripped the INDIRECT_REF with the r11-4758 change, we deduced
> 'rr's type as if decltype had gotten a name, resulting in 'int &&'.
> 
> I suspect I thought that the REF_PARENTHESIZED_P check when setting
> 'bool id' in do_auto_deduction would handle the (r) case, but that's not
> the case; while the documentation for REF_PARENTHESIZED_P specifically says
> it can be set in INDIRECT_REF, we don't actually do so.
> 
> This patch sets REF_PARENTHESIZED_P even on REFERENCE_REF_P, so that
> do_auto_deduction can use it.
> 
> It also removes code in maybe_undo_parenthesized_ref that I think is
> dead -- and we don't hit it while running dg.exp.  To adduce more data,
> it also looks dead here:
> https://splichal.eu/lcov/gcc/cp/semantics.c.gcov.html

Agreed, that code is dead since r9-1417.

> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk and 11?
> 
> 	PR c++/103403
> 
> gcc/cp/ChangeLog:
> 
> 	* cp-gimplify.c (cp_fold): Don't recurse if maybe_undo_parenthesized_ref
> 	doesn't change its argument.
> 	* pt.c (do_auto_deduction): Don't strip REFERENCE_REF_P trees.  Also
> 	REF_PARENTHESIZED_P for REFERENCE_REF_P.
> 	* semantics.c (force_paren_expr): Set REF_PARENTHESIZED_P on
> 	REFERENCE_REF_P trees too.
> 	(maybe_undo_parenthesized_ref): Remove dead code.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp1y/decltype-auto2.C: New test.
> 	* g++.dg/cpp1y/decltype-auto3.C: New test.
> ---
>   gcc/cp/cp-gimplify.c                        |  3 ++-
>   gcc/cp/pt.c                                 |  5 ++---
>   gcc/cp/semantics.c                          | 18 ++++--------------
>   gcc/testsuite/g++.dg/cpp1y/decltype-auto2.C | 12 ++++++++++++
>   gcc/testsuite/g++.dg/cpp1y/decltype-auto3.C | 12 ++++++++++++
>   5 files changed, 32 insertions(+), 18 deletions(-)
>   create mode 100644 gcc/testsuite/g++.dg/cpp1y/decltype-auto2.C
>   create mode 100644 gcc/testsuite/g++.dg/cpp1y/decltype-auto3.C
> 
> diff --git a/gcc/cp/cp-gimplify.c b/gcc/cp/cp-gimplify.c
> index 0a002db14e7..e3ede02a48e 100644
> --- a/gcc/cp/cp-gimplify.c
> +++ b/gcc/cp/cp-gimplify.c
> @@ -2421,7 +2421,8 @@ cp_fold (tree x)
>         if (REF_PARENTHESIZED_P (x))
>   	{
>   	  tree p = maybe_undo_parenthesized_ref (x);
> -	  return cp_fold (p);
> +	  if (p != x)
> +	    return cp_fold (p);
>   	}
>         goto unary;
>   
> diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
> index f4b9d9673fb..c5b41b57028 100644
> --- a/gcc/cp/pt.c
> +++ b/gcc/cp/pt.c
> @@ -29889,11 +29889,10 @@ 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)
> +		      || TREE_CODE (init) == SCOPE_REF
> +		      || REFERENCE_REF_P (init))
>   		     && !REF_PARENTHESIZED_P (init)));

This change seems wrong; not all references are id-expressions or member 
accesses.  For instance, a call to a function returning a reference is 
not.  I think we still want to depend on the TREE_CODE, but we need to 
look at the TREE_CODE of stripped_init, not init.

>         tree deduced = finish_decltype_type (init, id, complain);
>         deduced = canonicalize_type_argument (deduced, complain);
> diff --git a/gcc/cp/semantics.c b/gcc/cp/semantics.c
> index cd1956497f8..edba4b60e10 100644
> --- a/gcc/cp/semantics.c
> +++ b/gcc/cp/semantics.c
> @@ -2049,7 +2049,8 @@ force_paren_expr (tree expr, bool even_uneval)
>       return expr;
>   
>     if (TREE_CODE (expr) == COMPONENT_REF
> -      || TREE_CODE (expr) == SCOPE_REF)
> +      || TREE_CODE (expr) == SCOPE_REF
> +      || REFERENCE_REF_P (expr))
>       REF_PARENTHESIZED_P (expr) = true;
>     else if (DECL_P (tree_strip_any_location_wrapper (expr)))
>       {
> @@ -2072,19 +2073,8 @@ maybe_undo_parenthesized_ref (tree t)
>     if (cxx_dialect < cxx14)
>       return t;
>   
> -  if (INDIRECT_REF_P (t) && REF_PARENTHESIZED_P (t))
> -    {
> -      t = TREE_OPERAND (t, 0);
> -      while (TREE_CODE (t) == NON_LVALUE_EXPR
> -	     || TREE_CODE (t) == NOP_EXPR)
> -	t = TREE_OPERAND (t, 0);
> -
> -      gcc_assert (TREE_CODE (t) == ADDR_EXPR
> -		  || TREE_CODE (t) == STATIC_CAST_EXPR);
> -      t = TREE_OPERAND (t, 0);
> -    }
> -  else if ((TREE_CODE (t) == PAREN_EXPR || TREE_CODE (t) == VIEW_CONVERT_EXPR)
> -	     && REF_PARENTHESIZED_P (t))
> +  if ((TREE_CODE (t) == PAREN_EXPR || TREE_CODE (t) == VIEW_CONVERT_EXPR)
> +      && REF_PARENTHESIZED_P (t))
>       t = TREE_OPERAND (t, 0);
>   
>     return t;
> diff --git a/gcc/testsuite/g++.dg/cpp1y/decltype-auto2.C b/gcc/testsuite/g++.dg/cpp1y/decltype-auto2.C
> new file mode 100644
> index 00000000000..56e011e36f4
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp1y/decltype-auto2.C
> @@ -0,0 +1,12 @@
> +// PR c++/103403
> +// { dg-do compile { target c++14 } }
> +
> +template<typename T>
> +auto constexpr RtoL1(T&& r) -> decltype(auto) {
> +    return (r);
> +};
> +int main() {
> +    int t;
> +    int x{3};
> +    decltype (RtoL1(x+0)) y = t;
> +}
> diff --git a/gcc/testsuite/g++.dg/cpp1y/decltype-auto3.C b/gcc/testsuite/g++.dg/cpp1y/decltype-auto3.C
> new file mode 100644
> index 00000000000..914e87f5b79
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp1y/decltype-auto3.C
> @@ -0,0 +1,12 @@
> +// PR c++/103403
> +// { dg-do compile { target c++14 } }
> +
> +int main()
> +{
> +  int i = 1;
> +  int&& r = 1;
> +
> +  decltype(auto) ri = (i);
> +  decltype(auto) rr = (r);
> +  decltype((r)) rr2 = (r);
> +}
> 
> base-commit: 1e625a44f6f3001cea31e0f7c563943ecba92b68
>