Preserve using aliases in GetFullTypeName

Teach getFullyQualifiedType() to rebuild UsingType nodes instead of
desugaring them. This preserves the written alias while still fully
qualifying the underlying type, similar the existing TypedefType
handling.

Author: devajithvs

lib/Utils/AST.cpp

@@ -558,13 +558,6 @@ namespace TypeName {
     Qualifiers PrefixQualifiers = QT.getLocalQualifiers();
     QT = QualType(QT.getTypePtr(), 0);
 
-    // We don't consider the alias introduced by `using a::X` as a new type.
-    // The qualified name is still a::X.
-    if (const auto *UT = QT->getAs<UsingType>()) {
-      QT = Ctx.getQualifiedType(UT->desugar(), PrefixQualifiers);
-      return getFullyQualifiedType(QT, Ctx, WithGlobalNsPrefix);
-    }
-
     // Create a nested name specifier if needed.
     NestedNameSpecifier Prefix = createNestedNameSpecifierForScopeOf(
         Ctx, QT.getTypePtr(), true /*FullyQualified*/, WithGlobalNsPrefix);
@@ -583,6 +576,10 @@ namespace TypeName {
       QT = Ctx.getTypedefType(
           TT->getKeyword(), Prefix, TT->getDecl(),
           getFullyQualifiedType(TT->desugar(), Ctx, WithGlobalNsPrefix));
+    } else if (const auto* UT = dyn_cast<UsingType>(QT.getTypePtr())) {
+      QT = Ctx.getUsingType(UT->getKeyword(), Prefix, UT->getDecl(),
+                            getFullyQualifiedType(UT->desugar(), Ctx,
+                                                  WithGlobalNsPrefix));
     } else {
       assert(!Prefix && "Unhandled type node");
     }
@@ -674,6 +671,11 @@ namespace utils {
       QT = Ctx.getTypedefType(TT->getKeyword(), prefix, TT->getDecl(),
                               TypeName::getFullyQualifiedType(
                                   TT->desugar(), Ctx, WithGlobalNsPrefix));
+    } else if (const auto* UT = dyn_cast<UsingType>(QT.getTypePtr())) {
+      QT =
+          Ctx.getUsingType(UT->getKeyword(), prefix, UT->getDecl(),
+                           getFullyQualifiedType(UT->desugar(), Ctx,
+                                                 WithGlobalNsPrefix));
     }
     return QT;
   }