Re: [PATCH] c++: error with constexpr operator() [PR107939]

[Jason Merrill <jason@redhat.com>]

On 3/3/23 12:51, Marek Polacek wrote:
> Similarly to PR107938, this also started with r11-557, whereby cp_finish_decl
> can call check_initializer even in a template for a constexpr initializer.
> 
> Here we are rejecting
> 
>    extern const Q q;
> 
>    template<int>
>    constexpr auto p = q(0);
> 
> even though q has a constexpr operator().  It's deemed non-const by
> decl_maybe_constant_var_p because even though 'q' is const it is not
> of integral/enum type.  I think the fix is for p_c_e to treat q(0) as
> potentially-constant, as below.
> 
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk/12?
> 
> 	PR c++/107939
> 
> gcc/cp/ChangeLog:
> 
> 	* constexpr.cc (is_constexpr_function_object): New.
> 	(potential_constant_expression_1): Treat an object with constexpr
> 	operator() as potentially-constant.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp1y/var-templ74.C: Remove dg-error.
> 	* g++.dg/cpp1y/var-templ77.C: New test.
> ---
>   gcc/cp/constexpr.cc                      | 23 ++++++++++++++++++++++-
>   gcc/testsuite/g++.dg/cpp1y/var-templ74.C |  2 +-
>   gcc/testsuite/g++.dg/cpp1y/var-templ77.C | 14 ++++++++++++++
>   3 files changed, 37 insertions(+), 2 deletions(-)
>   create mode 100644 gcc/testsuite/g++.dg/cpp1y/var-templ77.C
> 
> diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
> index acf9847a4d1..7d786f332b4 100644
> --- a/gcc/cp/constexpr.cc
> +++ b/gcc/cp/constexpr.cc
> @@ -8929,6 +8929,24 @@ check_for_return_continue (tree *tp, int *walk_subtrees, void *data)
>     return NULL_TREE;
>   }
>   
> +/* Return true iff TYPE is a class with constexpr operator().  */
> +
> +static bool
> +is_constexpr_function_object (tree type)
> +{
> +  if (!CLASS_TYPE_P (type))
> +    return false;
> +
> +  for (tree f = TYPE_FIELDS (type); f; f = DECL_CHAIN (f))
> +    if (TREE_CODE (f) == FUNCTION_DECL
> +	&& DECL_OVERLOADED_OPERATOR_P (f)
> +	&& DECL_OVERLOADED_OPERATOR_IS (f, CALL_EXPR)
> +	&& DECL_DECLARED_CONSTEXPR_P (f))
> +      return true;
> +
> +  return false;
> +}
> +
>   /* Return true if T denotes a potentially constant expression.  Issue
>      diagnostic as appropriate under control of FLAGS.  If WANT_RVAL is true,
>      an lvalue-rvalue conversion is implied.  If NOW is true, we want to
> @@ -9160,7 +9178,10 @@ potential_constant_expression_1 (tree t, bool want_rval, bool strict, bool now,
>   	  }
>   	else if (fun)
>             {
> -	    if (RECUR (fun, rval))
> +	    if (VAR_P (fun)
> +		&& is_constexpr_function_object (TREE_TYPE (fun)))
> +	      /* Could be an object with constexpr operator().  */;

I guess if fun is not a function pointer, we don't know if we're using 
it as an lvalue or rvalue, so we want to pass 'any' for want_rval, which 
should make this work; I don't think we need to be specific about 
constexpr op(), as a constexpr conversion operator to fn* could also do 
the trick.

> +	    else if (RECUR (fun, rval))
>   	      /* Might end up being a constant function pointer.  */;
>   	    else
>   	      return false;
> diff --git a/gcc/testsuite/g++.dg/cpp1y/var-templ74.C b/gcc/testsuite/g++.dg/cpp1y/var-templ74.C
> index 4e2e800a6eb..c76a7d949ac 100644
> --- a/gcc/testsuite/g++.dg/cpp1y/var-templ74.C
> +++ b/gcc/testsuite/g++.dg/cpp1y/var-templ74.C
> @@ -9,7 +9,7 @@ struct Q {
>   extern const Q q;
>   
>   template<int>
> -constexpr const Q* p = q(0); // { dg-bogus "not usable" "PR107939" { xfail *-*-* } }
> +constexpr const Q* p = q(0);
>   
>   void
>   g ()
> diff --git a/gcc/testsuite/g++.dg/cpp1y/var-templ77.C b/gcc/testsuite/g++.dg/cpp1y/var-templ77.C
> new file mode 100644
> index 00000000000..b480f54b001
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp1y/var-templ77.C
> @@ -0,0 +1,14 @@
> +// PR c++/107939
> +// { dg-do compile { target c++14 } }
> +
> +struct Q {
> +  struct P {
> +    const Q* p;
> +  };
> +  int n;
> +  constexpr P operator()(int) const { return {this}; }
> +};
> +
> +extern const Q q;
> +template<int>
> +constexpr auto p = q(0);
> 
> base-commit: 9056d0df830c5a295d7594d517d409d10476990d