Re: [PATCH 1/2] c++: potentiality of templated memfn call [PR109480]

[Jason Merrill <jason@redhat.com>]

On 5/1/23 15:59, Patrick Palka wrote:
> Here we're incorrectly deeming the templated call a.g() inside b's
> initializer as potentially constant, despite g being non-constexpr,
> which leads to us wastefully instantiating the initializer ahead of time
> and triggering a bug in access checking deferral (which will get fixed
> in the subsequent patch).
> 
> This patch fixes this by calling get_fns earlier during potentiality
> checking so that we also handle the templated form of a member function
> call (whose overall callee is a COMPONENT_REF) when checking if the called
> function is constexpr etc.
> 
> 	PR c++/109480
> 
> gcc/cp/ChangeLog:
> 
> 	* constexpr.cc (potential_constant_expression_1) <case CALL_EXPR>:
> 	Reorganize to call get_fns sooner.  Remove dead store to 'fun'.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp0x/noexcept59.C: Make e() constexpr so that the
> 	expected "without object" diagnostic isn't replaced by a
> 	"call to non-constexpr function" diagnostic.
> 	* g++.dg/template/non-dependent25.C: New test.
> ---
>   gcc/cp/constexpr.cc                             | 16 ++++++++--------
>   gcc/testsuite/g++.dg/cpp0x/noexcept59.C         |  2 +-
>   gcc/testsuite/g++.dg/template/non-dependent25.C | 14 ++++++++++++++
>   3 files changed, 23 insertions(+), 9 deletions(-)
>   create mode 100644 gcc/testsuite/g++.dg/template/non-dependent25.C
> 
> diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
> index d1097764b10..29d872d0a5e 100644
> --- a/gcc/cp/constexpr.cc
> +++ b/gcc/cp/constexpr.cc
> @@ -9132,6 +9132,10 @@ potential_constant_expression_1 (tree t, bool want_rval, bool strict, bool now,
>   
>   	if (fun && is_overloaded_fn (fun))
>   	  {
> +	    if (!RECUR (fun, true))
> +	      return false;
> +	    fun = get_fns (fun);
> +
>   	    if (TREE_CODE (fun) == FUNCTION_DECL)
>   	      {
>   		if (builtin_valid_in_constant_expr_p (fun))
> @@ -9167,7 +9171,8 @@ potential_constant_expression_1 (tree t, bool want_rval, bool strict, bool now,
>   		   expression the address will be folded away, so look
>   		   through it now.  */
>   		if (DECL_NONSTATIC_MEMBER_FUNCTION_P (fun)
> -		    && !DECL_CONSTRUCTOR_P (fun))
> +		    && !DECL_CONSTRUCTOR_P (fun)
> +		    && !processing_template_decl)

I don't see any rationale for this hunk?

>   		  {
>   		    tree x = get_nth_callarg (t, 0);
>   		    if (is_this_parameter (x))
> @@ -9182,16 +9187,11 @@ potential_constant_expression_1 (tree t, bool want_rval, bool strict, bool now,
>   		    i = 1;
>   		  }
>   	      }
> -	    else
> -	      {
> -		if (!RECUR (fun, true))
> -		  return false;
> -		fun = get_first_fn (fun);
> -	      }
> +
> +	    fun = OVL_FIRST (fun);
>   	    /* Skip initial arguments to base constructors.  */
>   	    if (DECL_BASE_CONSTRUCTOR_P (fun))
>   	      i = num_artificial_parms_for (fun);
> -	    fun = DECL_ORIGIN (fun);
>   	  }
>   	else if (fun)
>             {
> diff --git a/gcc/testsuite/g++.dg/cpp0x/noexcept59.C b/gcc/testsuite/g++.dg/cpp0x/noexcept59.C
> index c752601ba09..1dc826d3111 100644
> --- a/gcc/testsuite/g++.dg/cpp0x/noexcept59.C
> +++ b/gcc/testsuite/g++.dg/cpp0x/noexcept59.C
> @@ -3,7 +3,7 @@
>   
>   template <class ...Ts> class A
>   {
> -  void e ();
> +  constexpr bool e () { return true; };
>     bool f (int() noexcept(this->e())); // { dg-error "this" }
>     bool g (int() noexcept(e()));	      // { dg-error "without object" }
>   };
> diff --git a/gcc/testsuite/g++.dg/template/non-dependent25.C b/gcc/testsuite/g++.dg/template/non-dependent25.C
> new file mode 100644
> index 00000000000..a2f9801e11f
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/template/non-dependent25.C
> @@ -0,0 +1,14 @@
> +// PR c++/109480
> +
> +template<class T>
> +struct A {
> +  void f() {
> +    A<int> a;
> +    const bool b = a.g();
> +  }
> +
> +private:
> +  bool g() const;
> +};
> +
> +template struct A<int>;