Hi!
C++26 is making in P2747R2 paper placement new constexpr.
While working on a patch for that, I've noticed we ICE starting with
GCC 14 on the following testcase.
The problem is that e.g. for the void * to sometype * casts checks,
we really assume the casts have their operand constant evaluated
as prvalue, but on the testcase the cast itself is evaluated with
vc_discard and that means op can end up e.g. a VAR_DECL which the
later code doesn't like and asserts on.
If the result type is void, we don't really need the cast operand
for anything, so can use vc_discard for the recursive call,
VIEW_CONVERT_EXPR can appear on the lhs, so we need to honor the
lval but otherwise the patch uses vc_prvalue.
I'd like to get this patch in before the rest of P2747R2 implementation,
so that it can be backported to 14.2 later on.
Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
2024-07-02 Jakub Jelinek <ja...@redhat.com>
Jason Merrill <ja...@redhat.com>
PR c++/115754
* constexpr.cc (cxx_eval_constant_expression) <case CONVERT_EXPR>:
For conversions to void, pass vc_discard to the recursive call
and otherwise for tcode other than VIEW_CONVERT_EXPR pass vc_prvalue.
* g++.dg/cpp26/pr115754.C: New test.
--- gcc/cp/constexpr.cc.jj 2024-05-08 10:16:53.966824718 +0200
+++ gcc/cp/constexpr.cc 2024-07-02 18:47:49.646843559 +0200
@@ -8103,7 +8103,10 @@ cxx_eval_constant_expression (const cons
tree oldop = TREE_OPERAND (t, 0);
tree op = cxx_eval_constant_expression (ctx, oldop,
- lval,
+ VOID_TYPE_P (TREE_TYPE (t))
+ ? vc_discard
+ : tcode == VIEW_CONVERT_EXPR
+ ? lval : vc_prvalue,
non_constant_p, overflow_p);
if (*non_constant_p)
return t;
--- gcc/testsuite/g++.dg/cpp26/pr115754.C.jj 2024-07-02 18:40:08.107750189
+0200
+++ gcc/testsuite/g++.dg/cpp26/pr115754.C 2024-07-02 18:39:57.110890921
+0200
@@ -0,0 +1,36 @@
+// PR c++/115754
+// { dg-do compile { target c++26 } }
+
+namespace std
+{
+ using size_t = decltype (sizeof 0);
+
+ template <typename T>
+ struct allocator
+ {
+ constexpr allocator () noexcept {}
+
+ constexpr T *allocate (size_t n)
+ { return static_cast<T *> (::operator new (n * sizeof(T))); }
+
+ constexpr void
+ deallocate (T *p, size_t n)
+ { ::operator delete (p); }
+ };
+}
+
+constexpr void *
+operator new (std::size_t, void *p) noexcept
+{ return p; }
+
+constexpr bool
+foo ()
+{
+ std::allocator<int> a;
+ auto b = a.allocate (1);
+ ::new (b) int ();
+ a.deallocate (b, 1);
+ return true;
+}
+
+constexpr bool a = foo ();
Jakub
