Fix crash when accessing property TemplateTypeParmDecl.DefaultArgument

* hasDefaultArgType() must be called first.
* Since the type may not have a default arg type, make the property nullable.

Author: ceresgalax

sources/ClangSharp/Cursors/Decls/TemplateTypeParmDecl.cs

@@ -5,13 +5,14 @@
 using ClangSharp.Interop;
 using static ClangSharp.Interop.CXCursorKind;
 using static ClangSharp.Interop.CX_DeclKind;
+using static ClangSharp.Interop.CXTypeKind;
 
 namespace ClangSharp;
 
 public sealed class TemplateTypeParmDecl : TypeDecl
 {
     private readonly Lazy<IReadOnlyList<Expr>> _associatedConstraints;
-    private readonly Lazy<Type> _defaultArgument;
+    private readonly Lazy<Type?> _defaultArgument;
 
     internal TemplateTypeParmDecl(CXCursor handle) : base(handle, CXCursor_TemplateTypeParameter, CX_DeclKind_TemplateTypeParm)
     {
@@ -27,12 +28,15 @@ internal TemplateTypeParmDecl(CXCursor handle) : base(handle, CXCursor_TemplateT
 
             return associatedConstraints;
         });
-        _defaultArgument = new Lazy<Type>(() => TranslationUnit.GetOrCreate<Type>(Handle.DefaultArgType));
+        _defaultArgument = new Lazy<Type?>(() => {
+            CXType defaultArgType = Handle.DefaultArgType;
+            return defaultArgType.kind == CXType_Invalid ? null : TranslationUnit.GetOrCreate<Type>(defaultArgType);
+        });
     }
 
     public IReadOnlyList<Expr> AssociatedConstraints => _associatedConstraints.Value;
 
-    public Type DefaultArgument => _defaultArgument.Value;
+    public Type? DefaultArgument => _defaultArgument.Value;
 
     public bool DefaultArgumentWasInherited => Handle.HasInheritedDefaultArg;
 

sources/libClangSharp/ClangSharp.cpp

@@ -1001,15 +1001,19 @@ CXCursor clangsharp_Cursor_getDefaultArg(CXCursor C) {
 }
 
 CXType clangsharp_Cursor_getDefaultArgType(CXCursor C) {
+    QualType QT;
+
     if (isDeclOrTU(C.kind)) {
         const Decl* D = getCursorDecl(C);
 
         if (const TemplateTypeParmDecl* TTPD = dyn_cast<TemplateTypeParmDecl>(D)) {
-            return MakeCXType(TTPD->getDefaultArgument(), getCursorTU(C));
+            if (TTPD->hasDefaultArgument()) {
+                QT = TTPD->getDefaultArgument();
+            }
         }
     }
 
-    return MakeCXType(QualType(), getCursorTU(C));
+    return MakeCXType(QT, getCursorTU(C));
 }
 
 CXCursor clangsharp_Cursor_getDefinition(CXCursor C) {