From fd94c6f239d2436b6152935b4c99217129953a5d Mon Sep 17 00:00:00 2001
From: Boris Kolpackov <boris@codesynthesis.com>
Date: Mon, 20 May 2024 12:23:58 +0200
Subject: Add convert_to_base<T>(value) variants that allow derive-to-base
 conversion

---
 libbuild2/variable.hxx | 15 +++++++++++----
 libbuild2/variable.txx | 35 +++++++++++++++++++++++++++++++++++
 2 files changed, 46 insertions(+), 4 deletions(-)

diff --git a/libbuild2/variable.hxx b/libbuild2/variable.hxx
index aed3350..6dfbbc6 100644
--- a/libbuild2/variable.hxx
+++ b/libbuild2/variable.hxx
@@ -92,6 +92,9 @@ namespace build2
     // static_cast to const T*. If it is NULL, then cast data_ directly. Note
     // that this function is used for both const and non-const values.
     //
+    // @@ This is currently ignored by as<T>() which is now used in quite a
+    //    few places (in particular, grep for as<T>).
+    //
     const void* (*const cast) (const value&, const value_type*);
 
     // If NULL, then the types are compared as PODs using memcmp().
@@ -702,14 +705,18 @@ namespace build2
   template <typename T> T convert (names&&);
 
   // Convert value to T. If value is already of type T, then simply cast it.
-  // Otherwise call convert(names) above. If value is NULL, then throw
-  // invalid_argument (with an appropriate message).
+  // Otherwise call convert(names) above. If the value is NULL, then throw
+  // invalid_argument (with an appropriate message). See also
+  // convert_to_base() below.
   //
   template <typename T> T convert (value&&);
+  template <typename T> T convert (const value&);
 
-  // As above but preserving the value.
+  // As above but also allow the derived-to-base conversions (where T is
+  // base). Note that this call may potentially slice the value.
   //
-  template <typename T> T convert (const value&);
+  template <typename T> T convert_to_base (value&&);
+  template <typename T> T convert_to_base (const value&);
 
   // Default implementations of the dtor/copy_ctor/copy_assing callbacks for
   // types that are stored directly in value::data_ and the provide all the
diff --git a/libbuild2/variable.txx b/libbuild2/variable.txx
index 0b831e9..a1ee340 100644
--- a/libbuild2/variable.txx
+++ b/libbuild2/variable.txx
@@ -38,6 +38,11 @@ namespace build2
     {
       if (v.type == nullptr)
         return convert<T> (move (v).as<names> ());
+      //
+      // Note that while it may be tempting to use is_a() here like in cast(),
+      // the implications are unclear (i.e., we may end up relaxing things we
+      // don't want to). So we have the convert_to_base() variants instead.
+      //
       else if (v.type == &value_traits<T>::value_type)
         return move (v).as<T> ();
     }
@@ -61,6 +66,36 @@ namespace build2
   }
 
   template <typename T>
+  T
+  convert_to_base (value&& v)
+  {
+    if (v)
+    {
+      if (v.type == nullptr)
+        return convert<T> (move (v).as<names> ());
+      else if (v.type->is_a<T> ())
+        return move (v).as<T> ();
+    }
+
+    convert_throw (v ? v.type : nullptr, value_traits<T>::value_type);
+  }
+
+  template <typename T>
+  T
+  convert_to_base (const value& v)
+  {
+    if (v)
+    {
+      if (v.type == nullptr)
+        return convert<T> (names (v.as<names> ()));
+      else if (v.type->is_a<T> ())
+        return v.as<T> ();
+    }
+
+    convert_throw (v ? v.type : nullptr, value_traits<T>::value_type);
+  }
+
+  template <typename T>
   void
   default_dtor (value& v)
   {
-- 
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