[cling][AST] 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

interpreter/cling/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;
   }

roottest/root/meta/naming/execResolveTypedef.cxx

@@ -14,7 +14,10 @@
 
 namespace A1 { namespace B2 { namespace C3 { typedef int what; } } }
 namespace NS { typedef int IntNS_t; }
-namespace SG { typedef std::uint32_t sgkey_t; }
+namespace SG {
+typedef std::uint32_t sgkey_t;
+using sgkey2_t = std::uint32_t;
+} // namespace SG
 namespace RT {
 namespace EX {
 struct ClusterSize {
@@ -27,6 +30,7 @@ using ClusterSize_t = ClusterSize;
 
 struct PackedParameters {
    SG::sgkey_t  m_sgkey;
+   SG::sgkey2_t m_sgkey2;
 };
 
 class Object
@@ -226,6 +230,7 @@ int execResolveTypedef()
    testing("unsigned int", TClassEdit::ResolveTypedef("SG::sgkey_t"));
    testing("unsigned int", TClass::GetClass("PackedParameters")->GetDataMember("m_sgkey")->GetTrueTypeName());
    testing("SG::sgkey_t", TClass::GetClass("PackedParameters")->GetDataMember("m_sgkey")->GetFullTypeName());
+   testing("SG::sgkey2_t", TClass::GetClass("PackedParameters")->GetDataMember("m_sgkey2")->GetFullTypeName());
    testing("RT::EX::ClusterSize", TClassEdit::ResolveTypedef("RT::EX::ClusterSize_t"));
    return 0;
 }

roottest/root/meta/naming/execResolveTypedef.ref

@@ -86,5 +86,6 @@ Test 83 The result is correct: ::int
 Test 84 The result is correct: unsigned int
 Test 85 The result is correct: unsigned int
 Test 86 The result is correct: SG::sgkey_t
-Test 87 The result is correct: RT::EX::ClusterSize
+Test 87 The result is correct: SG::sgkey2_t
+Test 88 The result is correct: RT::EX::ClusterSize
 (int) 0