[clang] [clang] Fix array types comparison in getCommonSugaredType (PR #131649)

[Matheus Izvekov via cfe-commits]

================
@@ -14171,6 +14171,15 @@ static QualType getCommonSugarTypeNode(ASTContext &Ctx, const Type *X,
 static auto unwrapSugar(SplitQualType &T, Qualifiers &QTotal) {
   SmallVector<SplitQualType, 8> R;
   while (true) {
+    if (const auto *ATy = dyn_cast<ArrayType>(T.Ty)) {
+      // C++ 9.3.3.4p3: Any type of the form "cv-qualifier-seq array of N U" is
+      // adjusted to "array of N cv-qualifier-seq U".
+      // C23 6.7.3p10: If the specification of an array type includes any type
+      // qualifiers, both the array and the element type are so-qualified.
+      //
+      // To simplify comparison remove the redundant qualifiers from the array.
+      T.Quals.removeCVRQualifiers(Qualifiers::Const | Qualifiers::Volatile);
----------------
mizvekov wrote:

This is at the wrong level to fix the problem.
We have the logic for dealing with Array qualifiers in `getCommonArrayElementType`.

That's where we adjust the outer qualifiers for Array types accordingly to the qualifiers of the element type.

Looks like there is a bug in the qualifier algebra there:
```C++
  QX += EX.getQualifiers() - RQ;
  QY += EY.getQualifiers() - RQ;
```
We want to break down those two lines to:
```C++
  QX += EX.getQualifiers();
  QY += EY.getQualifiers();
  QX -= RQ;
  QY -= RQ;
```

I believe this should fix your problem.

https://github.com/llvm/llvm-project/pull/131649