c++: constexpr evaluation of abi::__dynamic_cast [PR120620]

r13-3299 changed our internal declaration of __dynamic_cast to reside
inside the abi/__cxxabiv1:: namespace instead of the global namespace,
matching the real declaration.  This inadvertently made us now attempt
constexpr evaluation of user-written calls to abi::__dynamic_cast since
cxx_dynamic_cast_fn_p now also returns true for them, but we're not
prepared to handle arbitrary calls to __dynamic_cast, and therefore ICE.

This patch restores cxx_dynamic_cast_fn_p to return true only for
synthesized calls to __dynamic_cast, which can be distinguished by
DECL_ARTIFICIAL, since apparently the synthesized declaration of
__dynamic_cast doesn't get merged with the actual declaration.

	PR c++/120620

gcc/cp/ChangeLog:

	* constexpr.cc (cxx_dynamic_cast_fn_p): Return true only
	for synthesized __dynamic_cast.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/constexpr-dynamic19.C: New test.
	* g++.dg/cpp2a/constexpr-dynamic1a.C: New test.

Reviewed-by: Jason Merrill <[email protected]>

Author: Patrick Palka

gcc/cp/constexpr.cc

@@ -3240,7 +3240,11 @@ cxx_dynamic_cast_fn_p (tree fndecl)
 {
   return (cxx_dialect >= cxx20
 	  && id_equal (DECL_NAME (fndecl), "__dynamic_cast")
-	  && CP_DECL_CONTEXT (fndecl) == abi_node);
+	  && CP_DECL_CONTEXT (fndecl) == abi_node
+	  /* Only consider implementation-detail __dynamic_cast calls that
+	     correspond to a dynamic_cast, and ignore direct calls to
+	     abi::__dynamic_cast.  */
+	  && DECL_ARTIFICIAL (fndecl));
 }
 
 /* Often, we have an expression in the form of address + offset, e.g.

gcc/testsuite/g++.dg/cpp2a/constexpr-dynamic19.C

@@ -0,0 +1,10 @@
+// PR c++/120620
+// { dg-do compile }
+
+#include <cxxabi.h>
+
+struct A* a;
+
+void f() {
+  void* const p = abi::__dynamic_cast(&a, 0, 0, 42);
+}

gcc/testsuite/g++.dg/cpp2a/constexpr-dynamic1a.C

@@ -0,0 +1,6 @@
+// Test that including <cxxabi.h>, whence the actual abi:__dynamic_cast
+// is declared, doesn't affect our constexpr dynamic_cast handling.
+// { dg-do compile { target c++20 } }
+
+#include <cxxabi.h>
+#include "constexpr-dynamic1.C"