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

[Patrick Palka <ppalka@redhat.com>]

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?

-- >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>;
-- 
2.40.1.476.g69c786637d