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

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);
+	  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