fix: bad_expected_access::what() returns the error message

include/behaviortree_cpp/contrib/expected.hpp

@@ -258,6 +258,7 @@ namespace nonstd {
 #include <initializer_list>
 #include <memory>
 #include <new>
+#include <string>
 #include <system_error>
 #include <type_traits>
 #include <utility>
@@ -1607,9 +1608,10 @@ class bad_expected_access : public bad_expected_access< void >
     : m_error( error )
     {}
 
+    // BT.CPP patch (#1263): report the error message when the error is a string.
     virtual char const * what() const noexcept override
     {
-        return "bad_expected_access";
+        return what_of( m_error );
     }
 
     nsel_constexpr14 error_type & error() &
@@ -1637,6 +1639,17 @@ class bad_expected_access : public bad_expected_access< void >
 #endif
 
 private:
+    static char const * what_of( std::string const & error ) noexcept
+    {
+        return error.empty() ? "bad_expected_access" : error.c_str();
+    }
+
+    template< typename T >
+    static char const * what_of( T const & ) noexcept
+    {
+        return "bad_expected_access";
+    }
+
     error_type m_error;
 };
 

tests/gtest_basic_types.cpp

@@ -484,6 +484,49 @@ TEST(BasicTypes, Result_Error)
   ASSERT_EQ(result.error(), "error message");
 }
 
+TEST(BasicTypes, Expected_BadAccessMessage)
+{
+  auto bb = Blackboard::create();
+  try
+  {
+    (void)bb->getStamped<int>("missing_key").value();
+    FAIL() << "value() should throw";
+  }
+  catch(const std::exception& ex)
+  {
+    const std::string msg = ex.what();
+    EXPECT_NE(msg.find("Missing key [missing_key]"), std::string::npos) << msg;
+  }
+
+  Expected<int> res = nonstd::make_unexpected(std::string("error message"));
+  try
+  {
+    (void)res.value();
+    FAIL() << "value() should throw";
+  }
+  catch(const nonstd::bad_expected_access<std::string>& ex)
+  {
+    const auto copy = ex;
+    EXPECT_STREQ(copy.what(), "error message");
+    EXPECT_EQ(copy.error(), "error message");
+  }
+}
+
+TEST(BasicTypes, Expected_BadAccessMessageNotString)
+{
+  nonstd::expected<int, int> res = nonstd::make_unexpected(42);
+  try
+  {
+    (void)res.value();
+    FAIL() << "value() should throw";
+  }
+  catch(const nonstd::bad_expected_access<int>& ex)
+  {
+    EXPECT_STREQ(ex.what(), "bad_expected_access");
+    EXPECT_EQ(ex.error(), 42);
+  }
+}
+
 // ============ StringView tests ============
 
 TEST(BasicTypes, StringView_FromString)