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

[Jason Merrill]

On 5/3/23 16:50, Patrick Palka wrote:
> On Wed, 3 May 2023, Jason Merrill wrote:
> 
>> On 5/2/23 15:53, Patrick Palka wrote:
>>> on Tue, 2 May 2023, Patrick Palka wrote:
>>>
>>>> On Tue, 2 May 2023, Jason Merrill wrote:
>>>>
>>>>> 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?
>>>>
>>>> Now that we call get_fns earlier, we can reach this code path with a
>>>> templated non-static memfn call, but the code that follows assumes
>>>> non-templated form.
>>>>
>>>> I tried teaching it to handle the templated form too, but there's
>>>> apparently two different templated forms for non-static memfn calls,
>>>> one with a COMPONENT_REF callee and one with an ordinary BASELINK
>>>> callee (without a implicit object argument).  In the former the implict
>>>> object argument is inside the COMPONENT_REF (and is a reference instead
>>>> of a pointer), and in the latter we don't even have an implicit object
>>>> argument to inspect.
>>>>
>>>> FWIW I think which form we use depends on whether we know if the called
>>>> function is a member of the current instantiation, e.g
>>>>
>>>>     struct A { void f(); };
>>>>
>>>>     template<class T> struct B;
>>>>
>>>>     template<class T>
>>>>     struct C : B<T> {
>>>>       void g();
>>>>
>>>>       void h() {
>>>>         A::f(); // templated form has BASELINK callee, no object arg
>>>>         C::g(); // templated form has COMPONENT_REF callee
>>>>       }
>>>>     };
>>>>
>>>> So it seemed best to punt on templated non-static memfn calls here for
>>>> now and treat that as a separate enhancement.
>>>
>>> And I'm not even sure if the code path in question is necessary at all
>>> anymore: disabling it outright doesn't cause any regressions in the
>>> testsuite.
>>> It seems effectively equivalent to the body of the loop over the args a few
>>> lines later:
>>
>> If removing that hunk doesn't regress anything, let's do it.  Probably that
>> should have happened in r13-55-ge9d2adc17d0dbe
> 
> Sounds good, here's the combined patch which I'm bootstrapping for good
> measure.  Does it look OK for trunk if bootstrap+regtest succeeds?

OK.

> -- >8 --
> 
> Subject: [PATCH] c++: potentiality of templated memfn call [PR109480]
> 
> 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,
> which incidentally tiggers a bug in access checking deferral (to be
> fixed by the subsequent patch).
> 
> This patch fixes this by calling get_fns earlier during CALL_EXPR
> potentiality checking so that we're able to extract a FUNCTION_DECL out
> of a templated member function call (whose overall is typically a
> COMPONENT_REF) and to the usual checking if the called function is
> constexpr etc.
> 
> In passing, I noticed potential_constant_expression_1's special handling
> of the object argument of a non-static member function call is effectively
> the same as the generic argument handling a few lines later.  So this
> patch just gets rid of this special handling; otherwise we'd have to adapt
> it to handle templated versions of such calls.
> 
> 	PR c++/109480
> 
> gcc/cp/ChangeLog:
> 
> 	* constexpr.cc (potential_constant_expression_1) <case CALL_EXPR>:
> 	Reorganize to call get_fns sooner.  Remove special handling of
> 	the object argument of a non-static member function call.  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                           | 32 ++++---------------
>   gcc/testsuite/g++.dg/cpp0x/noexcept59.C       |  2 +-
>   .../g++.dg/template/non-dependent25.C         | 14 ++++++++
>   3 files changed, 21 insertions(+), 27 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..075339f7f62 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))
> @@ -9162,36 +9166,12 @@ potential_constant_expression_1 (tree t, bool want_rval, bool strict, bool now,
>   		      explain_invalid_constexpr_fn (fun);
>   		    return false;
>   		  }
> -		/* A call to a non-static member function takes the address
> -		   of the object as the first argument.  But in a constant
> -		   expression the address will be folded away, so look
> -		   through it now.  */
> -		if (DECL_NONSTATIC_MEMBER_FUNCTION_P (fun)
> -		    && !DECL_CONSTRUCTOR_P (fun))
> -		  {
> -		    tree x = get_nth_callarg (t, 0);
> -		    if (is_this_parameter (x))
> -		      return true;
> -		    /* Don't require an immediately constant value, as
> -		       constexpr substitution might not use the value.  */
> -		    bool sub_now = false;
> -		    if (!potential_constant_expression_1 (x, rval, strict,
> -							  sub_now, fundef_p,
> -							  flags, jump_target))
> -		      return false;
> -		    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..c00e44532b0
> --- /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(); // { dg-bogus "private" }
> +  }
> +
> +private:
> +  bool g() const;
> +};
> +
> +template struct A<int>;