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

[Jason Merrill <jason@redhat.com>]

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

>    for (; i < nargs; ++i)
>      {
>        tree x = get_nth_callarg (t, i);
>        /* In a template, reference arguments haven't been converted to
>           REFERENCE_TYPE and we might not even know if the parameter
>           is a reference, so accept lvalue constants too.  */
>        bool rv = processing_template_decl ? any : rval;
>        /* Don't require an immediately constant value, as constexpr
>           substitution might not use the value of the argument.  */
>        bool sub_now = false;
>        if (!potential_constant_expression_1 (x, rv, strict,
>                                              sub_now, fundef_p, flags,
>                                              jump_target))
>          return false;
>      }
> 
>>
>>>
>>>>    		  {
>>>>    		    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>;
>>>
>>>
>>
>