c++: Diagnose or avoid constexpr dtors in classes with virtual bases [PR114426]

I had another look at this P1 PR today.
You said in the "c++: fix in-charge parm in constexpr" mail back in December
(as well as in the r14-6507 commit message):
"Since a class with vbases can't have constexpr 'tors there isn't actually
a need for an in-charge parameter in a destructor" but the ICE is because
the destructor is marked implicitly constexpr.
https://eel.is/c++draft/dcl.constexpr#3.2 says that a destructor of a class
with virtual bases is not constexpr-suitable, but we were actually
implementing this just for constructors, so clearly my fault from the
https://wg21.link/P0784R7 implementation.  That paper clearly added that
sentence in there and removed similar sentence just from the constructor case.

So, the following patch makes sure the
  else if (CLASSTYPE_VBASECLASSES (DECL_CONTEXT (fun)))
    {
      ret = false;
      if (complain)
        error ("%q#T has virtual base classes", DECL_CONTEXT (fun));
    }
hunk is done no just for DECL_CONSTRUCTOR_P (fun), but also
DECL_DESTRUCTOR_P (fun) - in that case just for cxx_dialect >= cxx20,
as for cxx_dialect < cxx20 we already set ret = false; and diagnose
a different error, so no need to diagnose two.

2024-04-12  Jakub Jelinek  <jakub@redhat.com>

	PR c++/114426
	* constexpr.cc (is_valid_constexpr_fn): Return false/diagnose with
	complain destructors in classes with virtual bases.

	* g++.dg/cpp2a/pr114426.C: New test.
	* g++.dg/cpp2a/constexpr-dtor16.C: New test.

3 files changed, 21 insertions, 10 deletions

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index 02b34cc..fcc847d 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -262,18 +262,15 @@ is_valid_constexpr_fn (tree fun, bool complain)
 	inform (DECL_SOURCE_LOCATION (fun),
 		"lambdas are implicitly %<constexpr%> only in C++17 and later");
     }
-  else if (DECL_DESTRUCTOR_P (fun))
+  else if (DECL_DESTRUCTOR_P (fun) && cxx_dialect < cxx20)
     {
-      if (cxx_dialect < cxx20)
-	{
-	  ret = false;
-	  if (complain)
-	    error_at (DECL_SOURCE_LOCATION (fun),
-		      "%<constexpr%> destructors only available"
-		      " with %<-std=c++20%> or %<-std=gnu++20%>");
-	}
+      ret = false;
+      if (complain)
+	error_at (DECL_SOURCE_LOCATION (fun),
+		  "%<constexpr%> destructors only available with "
+		  "%<-std=c++20%> or %<-std=gnu++20%>");
     }
-  else if (!DECL_CONSTRUCTOR_P (fun))
+  else if (!DECL_CONSTRUCTOR_P (fun) && !DECL_DESTRUCTOR_P (fun))
     {
       tree rettype = TREE_TYPE (TREE_TYPE (fun));
       if (!literal_type_p (rettype))
diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-dtor16.C b/gcc/testsuite/g++.dg/cpp2a/constexpr-dtor16.C
new file mode 100644
index 0000000..b84aaf9
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-dtor16.C
@@ -0,0 +1,7 @@
+// PR c++/114426
+// { dg-do compile { target c++11 } }
+
+struct A { virtual ~A (); };
+struct B : virtual A { constexpr ~B () {} };
+// { dg-error "'struct B' has virtual base classes" "" { target c++20 } .-1 }
+// { dg-error "'constexpr' destructors only available with" "" { target c++17_down } .-2 }
diff --git a/gcc/testsuite/g++.dg/cpp2a/pr114426.C b/gcc/testsuite/g++.dg/cpp2a/pr114426.C
new file mode 100644
index 0000000..9000fa7
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/pr114426.C
@@ -0,0 +1,7 @@
+// PR c++/114426
+// { dg-do compile }
+// { dg-additional-options "-O2" }
+
+struct A { virtual ~A (); };
+struct B : virtual A { virtual void foo () = 0; };
+struct C : B { C () {} };