[clang] Fix array types comparison in getCommonSugaredType

Const-qualification of an array caused by constexpr specifier can
produce QualType, where the const qualifier is set both as fast
qualifier and as a qualifier of the array element type. It can result in
a compiler crash, because such QualType does not compare equal to the
same type but without extra qualification.

To avoid the crash, the redundant qualifiers are removed while searching
for common sugar.

It fixes #97005 (Clang
crashed in ASTContext::getCommonSugaredType).

Author: spavloff

clang/lib/AST/ASTContext.cpp

@@ -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);
+    }
     QTotal.addConsistentQualifiers(T.Quals);
     QualType NT = T.Ty->getLocallyUnqualifiedSingleStepDesugaredType();
     if (NT == QualType(T.Ty, 0))

clang/test/SemaCXX/constexpr-implicit-const-97005.cpp

@@ -0,0 +1,10 @@
+// RUN: %clang_cc1 -triple x86_64-linux-gnu %s
+
+bool a;
+constexpr const unsigned char c[] = { 5 };
+constexpr const unsigned char d[1] = { 0 };
+auto b = (a ? d : c);
+
+constexpr const unsigned char c1[][1] = {{ 5 }};
+constexpr const unsigned char d1[1][1] = {{ 0 }};
+auto b1 = (a ? d1 : c1);