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

[Patrick Palka <ppalka@redhat.com>]

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.

> 
> >   		  {
> >   		    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>;
> 
>