Re: [PATCH] c++, v2: Improve diagnostics for nullptr_t/info [PR126343]

On 7/27/26 4:40 PM, Jakub Jelinek wrote:
> On Fri, Jul 24, 2026 at 03:06:14PM -0400, Jason Merrill wrote:
>> On 7/22/26 4:44 AM, Jakub Jelinek wrote:
>>> We currently print std::nullptr_t or std::meta::info in diagnostics
>>> when seeing a NULLPTR_TYPE or META_TYPE, when they aren't type aliases
>>> (or when they are exactly those type aliases).
>>> I think that isn't a bad idea, the aliases is what users usually
>>> use for those.  There are 2 problems with this though.
>>> We print
>>> decltype(nullptr)
>>> and
>>> decltype(nullptr) const volatile
>>> exactly the same, both as std::nullptr_t, so the qualifiers are lost.
>>> And, e.g. in case of a static assertion failure when people want to find
>>> out why some reflections aren't equal we can print
>>> note: the comparison reduces to '(^^std::meta::info == ^^std::meta:info)'
>>> and the user then has no idea what is going on.  Is it because one of
>>> those is a type alias (which one), or because of cv-qual differences,
>>> or both?
>>> The following patch prints the aliases in normal %qT etc. printing
>>> only if unqualified, when qualified prints decltype(^^int) or
>>> decltype(nullptr) with the qualifications.
>>> And, when printing a reflection expression, it differentiates even
>>> between the type alias case and non-alias.
>>>
>>> So far lightly tested, ok for trunk if it passes full bootstrap/regtest?
>>>
>>> Or do you prefer always printing decltype?
>>
>> In the non-reflection case, we could continue to always print nullptr_t and
>> info, just add the cv-quals?
> 
> Ok, here is an updated version which does it that way.
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

OK.
> 2026-07-27  Jakub Jelinek  <[email protected]>
> 
> 	PR c++/126343
> 	* error.cc (dump_type) <case NULLPTR_TYPE>: Call
> 	pp_c_type_qualifier_list.
> 	(dump_type) <case META_TYPE>: Likewise.
> 	(dump_expr) <case REFLECT_EXPR>: For REFLECT_EXPR on
> 	non-typedef META_TYPE or NULLPTR_TYPE print
> 	decltype(^^int) or decltype(nullptr).
> 
> 	* g++.dg/reflect/pr126343.C: New test.
> 	* g++.dg/reflect/diag6.C: Adjust expected diagnostic wording.
> 	* g++.dg/reflect/init2.C: Likewise.
> 	* g++.dg/cpp0x/pr124489.C: Likewise.
> 
> --- gcc/cp/error.cc.jj	2026-07-22 11:41:35.683252926 +0200
> +++ gcc/cp/error.cc	2026-07-26 15:40:27.586800726 +0200
> @@ -876,10 +876,12 @@ dump_type (cxx_pretty_printer *pp, tree
>   
>       case NULLPTR_TYPE:
>         pp_cxx_ws_string (pp, "std::nullptr_t");
> +      pp_c_type_qualifier_list (pp, t);
>         break;
>   
>       case META_TYPE:
>         pp_cxx_ws_string (pp, "std::meta::info");
> +      pp_c_type_qualifier_list (pp, t);
>         break;
>   
>       case SPLICE_SCOPE:
> @@ -3481,7 +3483,24 @@ dump_expr (cxx_pretty_printer *pp, tree
>   	    if (DECL_P (h))
>   	      dump_decl (pp, h, flags);
>   	    else if (TYPE_P (h))
> -	      dump_type (pp, h, flags);
> +	      {
> +		/* For reflection we care about the difference
> +		   between std::meta::info/std::nullptr_t and
> +		   decltype(^^int)/decltype(nullptr).  */
> +		if (TREE_CODE (h) == META_TYPE && !typedef_variant_p (h))
> +		  {
> +		    pp_cxx_ws_string (pp, "decltype(^^int)");
> +		    pp_c_type_qualifier_list (pp, h);
> +		  }
> +		else if (TREE_CODE (h) == NULLPTR_TYPE
> +			 && !typedef_variant_p (h))
> +		  {
> +		    pp_cxx_ws_string (pp, "decltype(nullptr)");
> +		    pp_c_type_qualifier_list (pp, h);
> +		  }
> +		else
> +		  dump_type (pp, h, flags);
> +	      }
>   	    else
>   	      dump_expr (pp, h, flags);
>   	    break;
> --- gcc/testsuite/g++.dg/cpp0x/pr124489.C.jj	2026-03-27 10:17:15.510308285 +0100
> +++ gcc/testsuite/g++.dg/cpp0x/pr124489.C	2026-07-26 16:08:22.216399495 +0200
> @@ -4,6 +4,6 @@
>   void
>   g ()
>   {
> -  constexpr decltype(nullptr) dm = nullptr; // { dg-message ".constexpr std::nullptr_t dm. previously declared here" }
> -  constexpr decltype(nullptr) dm = nullptr; // { dg-error "redeclaration of .constexpr std::nullptr_t dm." }
> +  constexpr decltype(nullptr) dm = nullptr; // { dg-message ".constexpr std::nullptr_t const dm. previously declared here" }
> +  constexpr decltype(nullptr) dm = nullptr; // { dg-error "redeclaration of .constexpr std::nullptr_t const dm." }
>   }
> --- gcc/testsuite/g++.dg/reflect/pr126343.C.jj	2026-07-26 15:38:12.536398557 +0200
> +++ gcc/testsuite/g++.dg/reflect/pr126343.C	2026-07-26 15:38:12.536398557 +0200
> @@ -0,0 +1,43 @@
> +// PR c++/126343
> +// { dg-do compile { target c++26 } }
> +// { dg-additional-options "-freflection" }
> +namespace std {
> +  namespace meta {
> +    using info = decltype (^^::);
> +  }
> +  using nullptr_t = decltype (nullptr);
> +}
> +namespace my {
> +  using info = decltype (^^::) const;
> +  using nullptr_t = volatile decltype (nullptr);
> +}
> +constexpr auto a = ^^decltype (^^int);
> +constexpr auto b = ^^decltype (nullptr);
> +constexpr auto c = ^^const decltype (^^int);
> +constexpr auto d = ^^const decltype (nullptr);
> +constexpr auto e = ^^decltype (^^int) const volatile;
> +constexpr auto f = ^^decltype (nullptr) volatile;
> +constexpr auto g = ^^std::meta::info;
> +constexpr auto h = ^^std::nullptr_t;
> +constexpr auto i = ^^my::info;
> +constexpr auto j = ^^my::nullptr_t;
> +static_assert (a == ^^char);		// { dg-error "static assertion " }
> +					// { dg-message "note: the comparison reduces to '\\\(\\\^\\\^decltype\\\(\\\^\\\^int\\\) == \\\^\\\^char\\\)'" "" { target *-*-* } .-1 }
> +static_assert (b == ^^short);		// { dg-error "static assertion " }
> +					// { dg-message "note: the comparison reduces to '\\\(\\\^\\\^decltype\\\(nullptr\\\) == \\\^\\\^short int\\\)'" "" { target *-*-* } .-1 }
> +static_assert (c == ^^int);		// { dg-error "static assertion " }
> +					// { dg-message "note: the comparison reduces to '\\\(\\\^\\\^decltype\\\(\\\^\\\^int\\\) const == \\\^\\\^int\\\)'" "" { target *-*-* } .-1 }
> +static_assert (d == ^^long);		// { dg-error "static assertion " }
> +					// { dg-message "note: the comparison reduces to '\\\(\\\^\\\^decltype\\\(nullptr\\\) const == \\\^\\\^long int\\\)'" "" { target *-*-* } .-1 }
> +static_assert (e == ^^long long);	// { dg-error "static assertion " }
> +					// { dg-message "note: the comparison reduces to '\\\(\\\^\\\^decltype\\\(\\\^\\\^int\\\) const volatile == \\\^\\\^long long int\\\)'" "" { target *-*-* } .-1 }
> +static_assert (f == ^^unsigned char);	// { dg-error "static assertion " }
> +					// { dg-message "note: the comparison reduces to '\\\(\\\^\\\^decltype\\\(nullptr\\\) volatile == \\\^\\\^unsigned char\\\)'" "" { target *-*-* } .-1 }
> +static_assert (g == ^^unsigned short);	// { dg-error "static assertion " }
> +					// { dg-message "note: the comparison reduces to '\\\(\\\^\\\^std::meta::info == \\\^\\\^short unsigned int\\\)'" "" { target *-*-* } .-1 }
> +static_assert (h == ^^unsigned int);	// { dg-error "static assertion " }
> +					// { dg-message "note: the comparison reduces to '\\\(\\\^\\\^std::nullptr_t == \\\^\\\^unsigned int\\\)'" "" { target *-*-* } .-1 }
> +static_assert (i == ^^unsigned long);	// { dg-error "static assertion " }
> +					// { dg-message "note: the comparison reduces to '\\\(\\\^\\\^my::info == \\\^\\\^long unsigned int\\\)'" "" { target *-*-* } .-1 }
> +static_assert (j == ^^unsigned long long);// { dg-error "static assertion " }
> +					// { dg-message "note: the comparison reduces to '\\\(\\\^\\\^my::nullptr_t == \\\^\\\^long long unsigned int\\\)'" "" { target *-*-* } .-1 }
> --- gcc/testsuite/g++.dg/reflect/diag6.C.jj	2026-07-22 11:41:35.685252899 +0200
> +++ gcc/testsuite/g++.dg/reflect/diag6.C	2026-07-26 16:09:48.561296031 +0200
> @@ -5,6 +5,6 @@
>   void
>   g ()
>   {
> -  constexpr decltype(^^::) dm = ^^int; // { dg-message ".constexpr std::meta::info dm. previously declared here" }
> -  constexpr decltype(^^::) dm = ^^int; // { dg-error "redeclaration of .constexpr std::meta::info dm." }
> +  constexpr decltype(^^::) dm = ^^int; // { dg-message ".constexpr std::meta::info const dm. previously declared here" }
> +  constexpr decltype(^^::) dm = ^^int; // { dg-error "redeclaration of .constexpr std::meta::info const dm." }
>   }
> --- gcc/testsuite/g++.dg/reflect/init2.C.jj	2026-07-22 11:41:35.685252899 +0200
> +++ gcc/testsuite/g++.dg/reflect/init2.C	2026-07-26 16:10:21.208878807 +0200
> @@ -8,7 +8,7 @@ f ()
>     constexpr static auto srefl = ^^int;
>     constexpr auto *p = &srefl;
>     constexpr auto **q = &p;  // { dg-error "unable to deduce" }
> -  // { dg-message "types .auto\\*. and .std::meta::info\\* const." "" { target *-*-* } .-1 }
> +  // { dg-message "types .auto\\*. and .std::meta::info const\\* const." "" { target *-*-* } .-1 }
>   }
>   
>   void
> 
> 
> 	Jakub
>