From 095cffecbd6c34105468c42414cef445dc1c0998 Mon Sep 17 00:00:00 2001
From: Karen Arutyunov <karen@codesynthesis.com>
Date: Thu, 10 Jun 2021 21:43:56 +0300
Subject: Add path::try_relative()

---
 libbutl/path.mxx |  6 ++++++
 libbutl/path.txx | 16 +++++++++++++---
 2 files changed, 19 insertions(+), 3 deletions(-)

diff --git a/libbutl/path.mxx b/libbutl/path.mxx
index 12479ce..5a41ddc 100644
--- a/libbutl/path.mxx
+++ b/libbutl/path.mxx
@@ -952,6 +952,12 @@ LIBBUTL_MODEXPORT namespace butl
     basic_path
     relative (basic_path) const;
 
+    // As above but return nullopt rather than throw if a relative path cannot
+    // be derived.
+    //
+    optional<basic_path>
+    try_relative (basic_path) const;
+
     // Iteration over path components.
     //
     // Note that for an absolute POSIX path the first component is empty,
diff --git a/libbutl/path.txx b/libbutl/path.txx
index 45b62bd..84fc038 100644
--- a/libbutl/path.txx
+++ b/libbutl/path.txx
@@ -103,8 +103,8 @@ LIBBUTL_MODEXPORT namespace butl //@@ MOD Clang needs this for some reason.
 #endif
 
   template <typename C, typename K>
-  basic_path<C, K> basic_path<C, K>::
-  relative (basic_path<C, K> d) const
+  optional<basic_path<C, K>> basic_path<C, K>::
+  try_relative (basic_path<C, K> d) const
   {
     dir_type r;
 
@@ -118,12 +118,22 @@ LIBBUTL_MODEXPORT namespace butl //@@ MOD Clang needs this for some reason.
       // Roots of the paths do not match.
       //
       if (d.root ())
-        throw invalid_basic_path<C> (this->path_);
+        return nullopt;
     }
 
     return r / leaf (d);
   }
 
+  template <typename C, typename K>
+  basic_path<C, K> basic_path<C, K>::
+  relative (basic_path<C, K> d) const
+  {
+    if (optional<basic_path<C, K>> r = try_relative (std::move (d)))
+      return std::move (*r);
+
+    throw invalid_basic_path<C> (this->path_);
+  }
+
 #ifdef _WIN32
   // Find the actual spelling of a name in the specified dir. If the name is
   // found, append it to the result and return true. Otherwise, return false.
-- 
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