Re: [PATCH v2] c++: wrong error with static constexpr var in tmpl [PR109876]

[Jason Merrill <jason@redhat.com>]

On 7/13/23 14:54, Marek Polacek wrote:
> On Fri, May 26, 2023 at 09:47:10PM -0400, Jason Merrill wrote:
>> On 5/26/23 19:18, Marek Polacek wrote:
>>> The is_really_empty_class check is sort of non-obvious but the
>>> comment should explain why I added it.
>>>
>>> +	       /* When there's nothing to initialize, we'll never mark the
>>> +		  VAR_DECL TREE_CONSTANT, therefore it would remain
>>> +		  value-dependent and we wouldn't instantiate.  */
>   
> Sorry it's taken so long to get back to this.
> 
>> Interesting.  Can we change that (i.e. mark it TREE_CONSTANT) rather than
>> work around it here?
> 
> I think we can.  Maybe as in the below:
> 
> -- >8 --
> Since r8-509, we'll no longer create a static temporary var for
> the initializer '{ 1, 2 }' for num in the attached test because
> the code in finish_compound_literal is now guarded by
> '&& fcl_context == fcl_c99' but it's fcl_functional here.  This
> causes us to reject num as non-constant when evaluating it in
> a template.
> 
> Jason's idea was to treat num as value-dependent even though it
> actually isn't.  This patch implements that suggestion.
> 
> We weren't marking objects whose type is an empty class type
> constant.  This patch changes that so that v_d_e_p doesn't need
> to check is_really_empty_class.
> 
> Co-authored-by: Jason Merrill <jason@redhat.com>
> 
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

OK, thanks.

Incidentally, I prefer to put the "ok?" line above the scissors line 
since it isn't intended to be part of the commit message.

> 	PR c++/109876
> 
> gcc/cp/ChangeLog:
> 
> 	* decl.cc (cp_finish_decl): Set TREE_CONSTANT when initializing
> 	an object of empty class type.
> 	* pt.cc (value_dependent_expression_p) <case VAR_DECL>: Treat a
> 	constexpr-declared non-constant variable as value-dependent.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp0x/constexpr-template12.C: New test.
> 	* g++.dg/cpp1z/constexpr-template1.C: New test.
> 	* g++.dg/cpp1z/constexpr-template2.C: New test.
> ---
>   gcc/cp/decl.cc                                | 13 +++++--
>   gcc/cp/pt.cc                                  |  7 ++++
>   .../g++.dg/cpp0x/constexpr-template12.C       | 38 +++++++++++++++++++
>   .../g++.dg/cpp1z/constexpr-template1.C        | 25 ++++++++++++
>   .../g++.dg/cpp1z/constexpr-template2.C        | 25 ++++++++++++
>   5 files changed, 105 insertions(+), 3 deletions(-)
>   create mode 100644 gcc/testsuite/g++.dg/cpp0x/constexpr-template12.C
>   create mode 100644 gcc/testsuite/g++.dg/cpp1z/constexpr-template1.C
>   create mode 100644 gcc/testsuite/g++.dg/cpp1z/constexpr-template2.C
> 
> diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
> index 60f107d50c4..792ab330dd0 100644
> --- a/gcc/cp/decl.cc
> +++ b/gcc/cp/decl.cc
> @@ -8200,7 +8200,6 @@ void
>   cp_finish_decl (tree decl, tree init, bool init_const_expr_p,
>   		tree asmspec_tree, int flags)
>   {
> -  tree type;
>     vec<tree, va_gc> *cleanups = NULL;
>     const char *asmspec = NULL;
>     int was_readonly = 0;
> @@ -8220,7 +8219,7 @@ cp_finish_decl (tree decl, tree init, bool init_const_expr_p,
>     /* Parameters are handled by store_parm_decls, not cp_finish_decl.  */
>     gcc_assert (TREE_CODE (decl) != PARM_DECL);
>   
> -  type = TREE_TYPE (decl);
> +  tree type = TREE_TYPE (decl);
>     if (type == error_mark_node)
>       return;
>   
> @@ -8410,7 +8409,7 @@ cp_finish_decl (tree decl, tree init, bool init_const_expr_p,
>   	  if (decl_maybe_constant_var_p (decl)
>   	      /* FIXME setting TREE_CONSTANT on refs breaks the back end.  */
>   	      && !TYPE_REF_P (type))
> -	    TREE_CONSTANT (decl) = 1;
> +	    TREE_CONSTANT (decl) = true;
>   	}
>         /* This is handled mostly by gimplify.cc, but we have to deal with
>   	 not warning about int x = x; as it is a GCC extension to turn off
> @@ -8421,6 +8420,14 @@ cp_finish_decl (tree decl, tree init, bool init_const_expr_p,
>   	  && !warning_enabled_at (DECL_SOURCE_LOCATION (decl), OPT_Winit_self))
>   	suppress_warning (decl, OPT_Winit_self);
>       }
> +  else if (VAR_P (decl)
> +	   && COMPLETE_TYPE_P (type)
> +	   && !TYPE_REF_P (type)
> +	   && !dependent_type_p (type)
> +	   && is_really_empty_class (type, /*ignore_vptr*/false))
> +    /* We have no initializer but there's nothing to initialize anyway.
> +       Treat DECL as constant due to c++/109876.  */
> +    TREE_CONSTANT (decl) = true;
>   
>     if (flag_openmp
>         && TREE_CODE (decl) == FUNCTION_DECL
> diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
> index fa15b75b9c5..255d18b9539 100644
> --- a/gcc/cp/pt.cc
> +++ b/gcc/cp/pt.cc
> @@ -27983,6 +27983,13 @@ value_dependent_expression_p (tree expression)
>         else if (TYPE_REF_P (TREE_TYPE (expression)))
>   	/* FIXME cp_finish_decl doesn't fold reference initializers.  */
>   	return true;
> +      /* We have a constexpr variable and we're processing a template.  When
> +	 there's lifetime extension involved (for which finish_compound_literal
> +	 used to create a temporary), we'll not be able to evaluate the
> +	 variable until instantiating, so pretend it's value-dependent.  */
> +      else if (DECL_DECLARED_CONSTEXPR_P (expression)
> +	       && !TREE_CONSTANT (expression))
> +	return true;
>         return false;
>   
>       case DYNAMIC_CAST_EXPR:
> diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-template12.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-template12.C
> new file mode 100644
> index 00000000000..a9e065320c8
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-template12.C
> @@ -0,0 +1,38 @@
> +// PR c++/109876
> +// { dg-do compile { target c++11 } }
> +
> +using size_t = decltype(sizeof 0);
> +
> +namespace std {
> +template <class> struct initializer_list {
> +  const int *_M_array;
> +  size_t _M_len;
> +  constexpr size_t size() const { return _M_len; }
> +};
> +} // namespace std
> +
> +constexpr std::initializer_list<int> gnum{2};
> +
> +template <int> struct Array {};
> +template <int> void g()
> +{
> +  static constexpr std::initializer_list<int> num{2};
> +  static_assert(num.size(), "");
> +  Array<num.size()> ctx;
> +
> +  constexpr Array<1> num1{};
> +}
> +
> +template <int N>
> +struct Foo
> +{
> +  static constexpr std::initializer_list<int> num = { 1, 2 };
> +  static_assert(num.size(), "");
> +  Array<num.size()> ctx;
> +};
> +
> +void
> +f (Foo<5>)
> +{
> +  g<0>();
> +}
> diff --git a/gcc/testsuite/g++.dg/cpp1z/constexpr-template1.C b/gcc/testsuite/g++.dg/cpp1z/constexpr-template1.C
> new file mode 100644
> index 00000000000..58be046fd36
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp1z/constexpr-template1.C
> @@ -0,0 +1,25 @@
> +// PR c++/109876
> +// { dg-do compile { target c++17 } }
> +
> +struct Foo {};
> +template <const Foo&> struct X {};
> +
> +void g()
> +{
> +  static constexpr Foo foo;
> +  X<foo> x;
> +}
> +
> +template<int>
> +void f()
> +{
> +  static constexpr Foo foo;
> +  X<foo> x;
> +}
> +
> +void
> +h ()
> +{
> +  f<0>();
> +  f<1>();
> +}
> diff --git a/gcc/testsuite/g++.dg/cpp1z/constexpr-template2.C b/gcc/testsuite/g++.dg/cpp1z/constexpr-template2.C
> new file mode 100644
> index 00000000000..5a9a96ba60a
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp1z/constexpr-template2.C
> @@ -0,0 +1,25 @@
> +// PR c++/109876
> +// { dg-do compile { target c++17 } }
> +
> +struct Foo {};
> +template <const Foo&> struct X {};
> +
> +void g()
> +{
> +  static constexpr Foo foo{};
> +  X<foo> x;
> +}
> +
> +template<int>
> +void f()
> +{
> +  static constexpr Foo foo{};
> +  X<foo> x;
> +}
> +
> +void
> +h ()
> +{
> +  f<0>();
> +  f<1>();
> +}
> 
> base-commit: 834f34947b20b18696487fc5d2bccab5ea720351