Re: [PATCH] c++: Improve printing of null pointer to data member constants [PR126599]

On 8/4/26 3:46 AM, Jakub Jelinek wrote:
> Hi!
> 
> For null pointer to data member we use INTEGER_CST with OFFSET_TYPE
> and -1 value.  When that is printed in diagnostics or in
> display_string_of, it is printed as -1, which is confusing to users
> and an implementation detail (well, part of ABI
> https://itanium-cxx-abi.github.io/cxx-abi/abi.html#data-member-pointers
> A null data member pointer is represented as an offset of -1.
> ).
> Now, as the first testcase shows, if we have a non-null pointer to data
> member constant, it is actually PTRMEM_CST and is printed in a user-friendly
> way, the following patch just arranges to print the OFFSET_TYPE -1
> INTEGER_CST similarly.
> 
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
> 
> 2026-08-04  Jakub Jelinek  <[email protected]>
> 
> 	PR c++/126599
> 	* error.cc (dump_expr): Print OFFSET_TYPE -1 differently.
> 
> 	* g++.dg/diagnostic/ptrtomem5.C: New test.
> 	* g++.dg/reflect/display_string_of1.C: Add 2 new tests.
> 	* g++.dg/reflect/u8display_string_of1.C: Likewise.
> 
> --- gcc/cp/error.cc.jj	2026-07-31 08:37:07.905871693 +0200
> +++ gcc/cp/error.cc	2026-08-03 17:33:12.517674249 +0200
> @@ -2474,8 +2474,23 @@ dump_expr (cxx_pretty_printer *pp, tree
>   	pp_cxx_ws_string (pp, M_("<unknown>"));
>         break;
>   
> -    case VOID_CST:
>       case INTEGER_CST:
> +      if (TYPE_PTRDATAMEM_P (TREE_TYPE (t)) && integer_all_onesp (t))
> +	{
> +	  /* OFFSET_TYPE -1 is a null pointer to member.  */
> +	  if (flags & TFF_EXPR_IN_PARENS)
> +	    pp_cxx_left_paren (pp);
> +	  pp_cxx_left_paren (pp);
> +	  dump_type (pp, TREE_TYPE (t), flags);
> +	  pp_cxx_right_paren (pp);
> +	  pp->constant (cxx_dialect < cxx11 ? null_pointer_node : nullptr_node);

It looks like you don't test the C++98 case.

> +	  if (flags & TFF_EXPR_IN_PARENS)
> +	    pp_cxx_right_paren (pp);
> +	  break;
> +	}
> +      /* FALLTHRU */
> +
> +    case VOID_CST:
>       case REAL_CST:
>       case STRING_CST:
>       case COMPLEX_CST:
> --- gcc/testsuite/g++.dg/diagnostic/ptrtomem5.C.jj	2026-08-03 17:16:01.549113408 +0200
> +++ gcc/testsuite/g++.dg/diagnostic/ptrtomem5.C	2026-08-03 17:15:09.305755762 +0200
> @@ -0,0 +1,15 @@
> +// PR c++/126599
> +// { dg-do compile { target c++17 } }
> +
> +struct S { int s, t, u[3][3]; };
> +using A = int[3][3];
> +template <auto M>
> +void foo (int);
> +void
> +bar ()
> +{
> +  foo <(int S::*)nullptr> ();	// { dg-error "no matching function for call to 'foo<\\\(\\\(int S::\\\*\\\)nullptr\\\)>\\\(\\\)'" }
> +  foo <&S::s> ();		// { dg-error "no matching function for call to 'foo<\\\&S::s>\\\(\\\)'" }
> +  foo <&S::t> ();		// { dg-error "no matching function for call to 'foo<\\\&S::t>\\\(\\\)'" }
> +  foo <(A S::*)nullptr> ();	// { dg-error "no matching function for call to 'foo<\\\(\\\(int \\\(S::\\\*\\\)\\\[3\\\]\\\[3\\\]\\\)nullptr\\\)>\\\(\\\)'" }
> +}
> --- gcc/testsuite/g++.dg/reflect/display_string_of1.C.jj	2026-04-01 00:03:32.626089610 +0200
> +++ gcc/testsuite/g++.dg/reflect/display_string_of1.C	2026-08-03 17:22:28.415414943 +0200
> @@ -135,6 +135,8 @@ foo (int a, const long b, T c, int d[4],
>     static_assert (display_string_of (members_of (^^V4, ctx)[0]) == "V4& V4::operator+=(const V4&)");
>     static_assert (display_string_of (members_of (^^V5, ctx)[0]) == "V5::operator int()");
>     static_assert (display_string_of (^^operator""_a) == "int operator\"\"_a(const char*)");
> +  static_assert (display_string_of (reflect_constant ((int U::*)nullptr)) == "(int U::*)nullptr");
> +  static_assert (display_string_of (reflect_constant (&U::u)) == "&U::u");
>   }
>   
>   namespace NS5 {
> --- gcc/testsuite/g++.dg/reflect/u8display_string_of1.C.jj	2026-04-01 00:03:32.626244353 +0200
> +++ gcc/testsuite/g++.dg/reflect/u8display_string_of1.C	2026-08-03 17:24:08.416213148 +0200
> @@ -135,6 +135,8 @@ foo (int a, const long b, T c, int d[4],
>     static_assert (u8display_string_of (members_of (^^V4, ctx)[0]) == u8"V4& V4::operator+=(const V4&)");
>     static_assert (u8display_string_of (members_of (^^V5, ctx)[0]) == u8"V5::operator int()");
>     static_assert (u8display_string_of (^^operator""_a) == u8"int operator\"\"_a(const char*)");
> +  static_assert (u8display_string_of (reflect_constant ((int U::*)nullptr)) == u8"(int U::*)nullptr");
> +  static_assert (u8display_string_of (reflect_constant (&U::u)) == u8"&U::u");
>   }
>   
>   namespace NS5 {
> 
> 	Jakub
>