Fix ambiguous static_cast

include/nlohmann/detail/output/binary_writer.hpp

@@ -881,7 +881,7 @@ class binary_writer
                     for (size_t i = 0; i < j.m_data.m_value.binary->size(); ++i)
                     {
                         oa->write_character(to_char_type(bjdata_draft3 ? 'B' : 'U'));
-                        oa->write_character(j.m_data.m_value.binary->data()[i]);
+                        oa->write_character(to_char_type(j.m_data.m_value.binary->data()[i]));
                     }
                 }
 
@@ -1634,7 +1634,9 @@ class binary_writer
         };
 
         string_t key = "_ArrayType_";
-        auto it = bjdtype.find(static_cast<string_t>(value.at(key)));
+        // use get<string_t>() instead of static_cast<string_t> to avoid an
+        // ambiguous conversion under explicit instantiation on C++17 (see #4825)
+        auto it = bjdtype.find(value.at(key).template get<string_t>());
         if (it == bjdtype.end())
         {
             return true;

include/nlohmann/json.hpp

@@ -4280,7 +4280,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         }
     };
 
-    data m_data = {};
+    data m_data = {}; // NOLINT(readability-redundant-member-init)
 
 #if JSON_DIAGNOSTICS
     /// a pointer to a parent value (for debugging purposes)

single_include/nlohmann/json.hpp

@@ -17076,7 +17076,7 @@ class binary_writer
                     for (size_t i = 0; i < j.m_data.m_value.binary->size(); ++i)
                     {
                         oa->write_character(to_char_type(bjdata_draft3 ? 'B' : 'U'));
-                        oa->write_character(j.m_data.m_value.binary->data()[i]);
+                        oa->write_character(to_char_type(j.m_data.m_value.binary->data()[i]));
                     }
                 }
 
@@ -17829,7 +17829,9 @@ class binary_writer
         };
 
         string_t key = "_ArrayType_";
-        auto it = bjdtype.find(static_cast<string_t>(value.at(key)));
+        // use get<string_t>() instead of static_cast<string_t> to avoid an
+        // ambiguous conversion under explicit instantiation on C++17 (see #4825)
+        auto it = bjdtype.find(value.at(key).template get<string_t>());
         if (it == bjdtype.end())
         {
             return true;
@@ -24798,7 +24800,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         }
     };
 
-    data m_data = {};
+    data m_data = {}; // NOLINT(readability-redundant-member-init)
 
 #if JSON_DIAGNOSTICS
     /// a pointer to a parent value (for debugging purposes)

tests/src/unit-regression2.cpp

@@ -66,7 +66,17 @@ using ordered_json = nlohmann::ordered_json;
     #endif
 #endif
 
+/////////////////////////////////////////////////////////////////////
+// for #4825 - explicitly instantiating basic_json must compile; this
+// forces instantiation of binary_writer::write_bjdata_ndarray, whose
+// static_cast<string_t> was ambiguous under explicit instantiation on
+// C++17. Merely compiling this translation unit is the regression test.
+/////////////////////////////////////////////////////////////////////
+template class nlohmann::basic_json<>;
+
+/////////////////////////////////////////////////////////////////////
 // for #4440
+/////////////////////////////////////////////////////////////////////
 #if JSON_HAS_RANGES == 1
     #include <ranges>
 #endif