c++: Fix ICE on constexpr placement new [PR115754]

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.

2024-07-02  Jakub Jelinek  <jakub@redhat.com>
	    Jason Merrill  <jason@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

@@ -8103,7 +8103,10 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
 	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

@@ -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 ();