[gcc r13-3638] c++: constexpr error with defaulted virtual dtor [PR93413]

[Patrick Palka <ppalka@gcc.gnu.org>]

https://gcc.gnu.org/g:c0b38e9fec71dc4cc5e99b1f0cb159c15a8da9ba

commit r13-3638-gc0b38e9fec71dc4cc5e99b1f0cb159c15a8da9ba
Author: Patrick Palka <ppalka@redhat.com>
Date:   Thu Nov 3 14:55:35 2022 -0400

    c++: constexpr error with defaulted virtual dtor [PR93413]
    
    We're rejecting the below testcase with
    
      error: 'virtual constexpr Base::~Base()' used before its definition
      error: 'virtual constexpr Derived::~Derived()' used before its definition
    
    due to special handling in mark_used added by r181272 to defer synthesis
    of virtual destructors until EOF (where we can set their linkage), which
    in turn makes them effectively unusable during constexpr evaluation.
    
    Fortunately it seems this special handling is unnecessary ever since
    r208030 enabled us to tentatively set linkage of all defaulted virtual
    destructors, including templated ones.  So this patch gets rid of this
    special handling.
    
            PR c++/93413
    
    gcc/cp/ChangeLog:
    
            * decl2.cc (mark_used): Don't defer synthesis of virtual
            functions.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/cpp2a/constexpr-virtual21.C: New test.

Diff:
---
 gcc/cp/decl2.cc                                  |  8 --------
 gcc/testsuite/g++.dg/cpp2a/constexpr-virtual21.C | 10 ++++++++++
 2 files changed, 10 insertions(+), 8 deletions(-)

diff --git a/gcc/cp/decl2.cc b/gcc/cp/decl2.cc
index e6779268ad4..eeb59eae64f 100644
--- a/gcc/cp/decl2.cc
+++ b/gcc/cp/decl2.cc
@@ -5788,14 +5788,6 @@ mark_used (tree decl, tsubst_flags_t complain /* = tf_warning_or_error */)
       && !DECL_DEFAULTED_OUTSIDE_CLASS_P (decl)
       && ! DECL_INITIAL (decl))
     {
-      /* Defer virtual destructors so that thunks get the right
-	 linkage.  */
-      if (DECL_VIRTUAL_P (decl) && !at_eof)
-	{
-	  note_vague_linkage_fn (decl);
-	  return true;
-	}
-
       /* Remember the current location for a function we will end up
 	 synthesizing.  Then we can inform the user where it was
 	 required in the case of error.  */
diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-virtual21.C b/gcc/testsuite/g++.dg/cpp2a/constexpr-virtual21.C
new file mode 100644
index 00000000000..8b70c5f538b
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-virtual21.C
@@ -0,0 +1,10 @@
+// PR c++/93413
+// { dg-do compile { target c++20 } }
+
+struct Base {
+  virtual ~Base() = default;
+};
+constexpr Base b;
+
+struct Derived : Base { };
+constexpr Derived d;