[clang][bytecode] Handle negative array sizes in constexpr new instead of asserting (llvm#155737)

This patch fixes a crash in the constexpr interpreter when evaluating
`new T[n]` with a negative element count.

Fixes llvm#152904

Author: snarang181

clang/lib/AST/ByteCode/Interp.h

@@ -3490,7 +3490,15 @@ inline bool AllocN(InterpState &S, CodePtr OpPC, PrimType T, const Expr *Source,
     S.Stk.push<Pointer>(0, nullptr);
     return true;
   }
-  assert(NumElements.isPositive());
+  if (NumElements.isNegative()) {
+    if (!IsNoThrow) {
+      S.FFDiag(S.Current->getSource(OpPC), diag::note_constexpr_new_negative)
+          << NumElements.toDiagnosticString(S.getASTContext());
+      return false;
+    }
+    S.Stk.push<Pointer>(0, nullptr);
+    return true;
+  }
 
   if (!CheckArraySize(S, OpPC, static_cast<uint64_t>(NumElements)))
     return false;

clang/test/AST/ByteCode/new-delete.cpp

@@ -1069,6 +1069,28 @@ namespace BaseCompare {
   static_assert(foo());
 }
 
+
+namespace NegativeArraySize { 
+  constexpr void f() { // both-error {{constexpr function never produces a constant expression}}
+    int x = -1;
+    int *p = new int[x]; //both-note {{cannot allocate array; evaluated array bound -1 is negative}} 
+  }
+} // namespace NegativeArraySize
+
+namespace NewNegSizeNothrow {
+  constexpr int get_neg_size() {
+    return -1;
+  }
+
+  constexpr bool test_nothrow_neg_size() {
+    int x = get_neg_size();
+    int* p = new (std::nothrow) int[x]; 
+    return p == nullptr;
+  }
+
+  static_assert(test_nothrow_neg_size(), "expected nullptr");
+} // namespace NewNegSizeNothrow
+
 #else
 /// Make sure we reject this prior to C++20
 constexpr int a() { // both-error {{never produces a constant expression}}