refactor: reimplement ADT/Polymorphic

This is a simpler design, with less differences in API wrt to
std::polymorphic, though still with some differences, includinding
nullability, and some unused features which are not important to us
are left out.

This avoids memory leaks facilitated by the previous design, and is
a step towards making the project pass the LeakSanitizer.

Author: mizvekov

include/mrdocs/ADT/Polymorphic.hpp

@@ -259,7 +259,9 @@ class MRDOCS_VISIBLE
                         {
                             return std::is_lt(cmp);
                         }
-                        return std::is_lt(CompareDerived(lhsInfo, rhsInfo));
+                        return std::is_lt(
+                            CompareDerived(Polymorphic<Info>(lhsInfo),
+                                           Polymorphic<Info>(rhsInfo)));
                     });
                 if (!opts.skipInherited)
                 {

include/mrdocs/ADT/detail/Polymorphic.hpp

@@ -29,7 +29,7 @@ struct EnumInfo final
     // Set too nonempty to the type when this is an explicitly typed enum. For
     //   enum Foo : short { ... };
     // this will be "short".
-    Polymorphic<TypeInfo> UnderlyingType;
+    Polymorphic<TypeInfo> UnderlyingType = std::nullopt;
 
     /** The members of this scope.
 

include/mrdocs/Corpus.hpp

@@ -31,7 +31,7 @@ struct FriendInfo final
 
     /** Befriended type.
     */
-    Polymorphic<TypeInfo> Type;
+    Polymorphic<TypeInfo> Type = std::nullopt;
 };
 
 MRDOCS_DECL

include/mrdocs/Metadata/Info/Enum.hpp

@@ -109,7 +109,7 @@ struct Param final
 {
     /** The type of this parameter
      */
-    Polymorphic<TypeInfo> Type;
+    Polymorphic<TypeInfo> Type = std::nullopt;
 
     /** The parameter name.
      */
@@ -154,7 +154,7 @@ struct FunctionInfo final
     : InfoCommonBase<InfoKind::Function>
 {
     /// Info about the return type of this function.
-    Polymorphic<TypeInfo> ReturnType;
+    Polymorphic<TypeInfo> ReturnType = std::nullopt;
 
     /// List of parameters.
     std::vector<Param> Params;

include/mrdocs/Metadata/Info/Friend.hpp

@@ -31,7 +31,7 @@ struct GuideInfo final
 
         This is always a SpecializationTypeInfo.
     */
-    Polymorphic<TypeInfo> Deduced;
+    Polymorphic<TypeInfo> Deduced = std::nullopt;
 
     /** Template head, if any.
     */

include/mrdocs/Metadata/Info/Function.hpp

@@ -32,7 +32,7 @@ struct OverloadsInfo final
     std::vector<SymbolID> Members;
 
     /// Info about the return type of this function.
-    Polymorphic<TypeInfo> ReturnType;
+    Polymorphic<TypeInfo> ReturnType = std::nullopt;
 
     //--------------------------------------------
 

include/mrdocs/Metadata/Info/Guide.hpp

@@ -25,7 +25,7 @@ namespace clang::mrdocs {
 struct TypedefInfo final
     : InfoCommonBase<InfoKind::Typedef>
 {
-    Polymorphic<TypeInfo> Type;
+    Polymorphic<TypeInfo> Type = std::nullopt;
 
     /** Indicates if this is a new C++ "using"-style typedef
 

include/mrdocs/Metadata/Info/Overloads.hpp

@@ -31,7 +31,7 @@ struct VariableInfo final
     : InfoCommonBase<InfoKind::Variable>
 {
     /** The type of the variable */
-    Polymorphic<TypeInfo> Type;
+    Polymorphic<TypeInfo> Type = std::nullopt;
 
     std::optional<TemplateInfo> Template;