Re: [PATCH] c++: address of NTTP object as targ [PR113242]

[Patrick Palka <ppalka@redhat.com>]

On Mon, 15 Jan 2024, Jason Merrill wrote:

> On 1/5/24 11:50, Patrick Palka wrote:
> > Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK
> > for trunk and perhaps 13?
> > 
> > -- >8 --
> > 
> > invalid_tparm_referent_p was rejecting using the address of a class NTTP
> > object as a template argument, but this should be fine.
> 
> Hmm, I suppose so; https://eel.is/c++draft/temp#param-8 saying "No two
> template parameter objects are template-argument-equivalent" suggests there
> can be only one.  And clang/msvc allow it.
> 
> > 	PR c++/113242
> > 
> > gcc/cp/ChangeLog:
> > 
> > 	* pt.cc (invalid_tparm_referent_p) <case ADDR_EXPR>: Suppress
> > 	DECL_ARTIFICIAL rejection test for class NTTP objects.
> > 
> > gcc/testsuite/ChangeLog:
> > 
> > 	* g++.dg/cpp2a/nontype-class61.C: New test.
> > ---
> >   gcc/cp/pt.cc                                 |  3 ++-
> >   gcc/testsuite/g++.dg/cpp2a/nontype-class61.C | 27 ++++++++++++++++++++
> >   2 files changed, 29 insertions(+), 1 deletion(-)
> >   create mode 100644 gcc/testsuite/g++.dg/cpp2a/nontype-class61.C
> > 
> > diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
> > index 154ac76cb65..8c7d178328d 100644
> > --- a/gcc/cp/pt.cc
> > +++ b/gcc/cp/pt.cc
> > @@ -7219,7 +7219,8 @@ invalid_tparm_referent_p (tree type, tree expr,
> > tsubst_flags_t complain)
> >   	   * a string literal (5.13.5),
> >   	   * the result of a typeid expression (8.2.8), or
> >   	   * a predefined __func__ variable (11.4.1).  */
> > -	else if (VAR_P (decl) && DECL_ARTIFICIAL (decl))
> > +	else if (VAR_P (decl) && !DECL_NTTP_OBJECT_P (decl)
> > +		 && DECL_ARTIFICIAL (decl))
> 
> If now some artificial variables are OK and others are not, perhaps we should
> enumerate them either way and abort if it's one we haven't specifically
> considered.

Sounds good, like so?  Shall we backport this patch or the original
patch to the 13 branch?

-- >8 --

Subject: [PATCH] c++: address of class NTTP object as targ [PR113242]

invalid_tparm_referent_p was rejecting using the address of a class NTTP
object as a template argument, but this should be fine.

This patch fixes this by refining the DECL_ARTIFICIAL rejection test to
check specifically for the kinds of artificial variables we want to
exclude.

	PR c++/113242

gcc/cp/ChangeLog:

	* pt.cc (invalid_tparm_referent_p) <case ADDR_EXPR>: Refine
	DECL_ARTIFICIAL rejection test.  Assert that C++20 template
	parameter objects are the only artificial variables we accept.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/nontype-class61.C: New test.
---
 gcc/cp/pt.cc                                 | 13 +++++++---
 gcc/testsuite/g++.dg/cpp2a/nontype-class61.C | 27 ++++++++++++++++++++
 2 files changed, 37 insertions(+), 3 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/nontype-class61.C

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index b6117231de1..885c297450e 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -7212,12 +7212,14 @@ invalid_tparm_referent_p (tree type, tree expr, tsubst_flags_t complain)
 	/* C++17: For a non-type template-parameter of reference or pointer
 	   type, the value of the constant expression shall not refer to (or
 	   for a pointer type, shall not be the address of):
-	   * a subobject (4.5),
+	   * a subobject (4.5), (relaxed in C++20)
 	   * a temporary object (15.2),
-	   * a string literal (5.13.5),
+	   * a string literal (5.13.5), (we diagnose this early in
+	     convert_nontype_argument)
 	   * the result of a typeid expression (8.2.8), or
 	   * a predefined __func__ variable (11.4.1).  */
-	else if (VAR_P (decl) && DECL_ARTIFICIAL (decl))
+	else if (VAR_P (decl)
+		 && (DECL_TINFO_P (decl) || DECL_FNAME_P (decl)))
 	  {
 	    if (complain & tf_error)
 	      error ("the address of %qD is not a valid template argument",
@@ -7242,6 +7244,11 @@ invalid_tparm_referent_p (tree type, tree expr, tsubst_flags_t complain)
 		     decl);
 	    return true;
 	  }
+
+	/* The only artificial variables we do accept are C++20
+	   template parameter objects.   */
+	if (VAR_P (decl) && DECL_ARTIFICIAL (decl))
+	  gcc_checking_assert (DECL_NTTP_OBJECT_P (decl));
       }
       break;
 
diff --git a/gcc/testsuite/g++.dg/cpp2a/nontype-class61.C b/gcc/testsuite/g++.dg/cpp2a/nontype-class61.C
new file mode 100644
index 00000000000..90805a05ecf
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/nontype-class61.C
@@ -0,0 +1,27 @@
+// PR c++/113242
+// { dg-do compile { target c++20 } }
+
+struct wrapper {
+  int n;
+};
+
+template<const wrapper& X>
+void f1() {
+  static_assert(X.n == 42);
+}
+
+template<const wrapper* X>
+void f2() {
+  static_assert(X->n == 42);
+}
+
+template<wrapper X>
+void g() {
+  f1<X>();
+  f2<&X>();
+}
+
+int main() {
+  constexpr wrapper X = {42};
+  g<X>();
+}
-- 
2.43.0.367.g186b115d30