[clang] [C2y] Implement WG14 N3369 and N3469 (_Countof) (PR #133125)

[Aaron Ballman via cfe-commits]

================
@@ -14926,6 +14926,42 @@ bool IntExprEvaluator::VisitUnaryExprOrTypeTraitExpr(
 
     return false;
   }
+  case UETT_CountOf: {
+    QualType Ty = E->getTypeOfArgument();
+    assert(Ty->isArrayType());
+
+    // We don't need to worry about array element qualifiers, so getting the
+    // unsafe array type is fine.
+    if (const auto *CAT =
+            dyn_cast<ConstantArrayType>(Ty->getAsArrayTypeUnsafe())) {
+      return Success(CAT->getSize(), E);
+    }
+
+    assert(!Ty->isConstantSizeType());
+
+    // If it's a variable-length array type, we need to check whether it is a
+    // multidimensional array. If so, we need to check the size expression of
+    // the VLA to see if it's a constant size. If so, we can return that value.
+    const auto *VAT = Info.Ctx.getAsVariableArrayType(Ty);
+    assert(VAT);
+    if (VAT->getElementType()->isArrayType()) {
+      std::optional<APSInt> Res =
+          VAT->getSizeExpr()->getIntegerConstantExpr(Info.Ctx);
+      if (Res) {
+        // The resulting value always has type size_t, so we need to make the
+        // returned APInt have the correct sign and bit-width.
+        APInt Val{
+            static_cast<unsigned>(Info.Ctx.getTypeSize(Info.Ctx.getSizeType())),
+            Res->getZExtValue()};
+        return Success(Val, E);
+      }
+    }
----------------
AaronBallman wrote:

See `test_constant_expression_behavior()`: 
```
  // Note that this applies to each array dimension.
  int another_array[n][7];
  static_assert(_Countof(another_array)); // expected-error {{static assertion expression is not an integral constant expression}}
  static_assert(_Countof(*another_array) == 7);

  // Only the first dimension is needed for constant evaluation; other
  // dimensions can be ignored.
  int yet_another_array[7][n];
  static_assert(_Countof(yet_another_array) == 7);
  static_assert(_Countof(*yet_another_array)); // expected-error {{static assertion expression is not an integral constant expression}}
```
Without this code, `static_assert(_Countof(yet_another_array) == 7);` would give an error that the assertion is not an ICE.

https://github.com/llvm/llvm-project/pull/133125