Take the natvis template from the PR in the amalgamation check

The check ran generate_natvis.py from a develop checkout, which loads
nlohmann_json.natvis.j2 from its own directory. A PR that changes the
template was therefore checked against develop's template and always
failed. Copy the PR's template next to the develop script before
running it.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

.github/workflows/check_amalgamation.yml

@@ -149,6 +149,13 @@ jobs:
           VERSION_MAJOR=$(grep -m1 'define NLOHMANN_JSON_VERSION_MAJOR' $ABI_MACROS | grep -o '[0-9]\+')
           VERSION_MINOR=$(grep -m1 'define NLOHMANN_JSON_VERSION_MINOR' $ABI_MACROS | grep -o '[0-9]\+')
           VERSION_PATCH=$(grep -m1 'define NLOHMANN_JSON_VERSION_PATCH' $ABI_MACROS | grep -o '[0-9]\+')
+          # The template is what this check verifies the committed natvis
+          # file against, so take it from this PR rather than from the
+          # develop checkout: generate_natvis.py loads the template from its
+          # own directory, and with develop's copy a PR that changes the
+          # template could never pass. Like the macro_builder step above,
+          # this only renders PR content in a job without credentials.
+          cp tools/generate_natvis/nlohmann_json.natvis.j2 $NATVIS_TOOL_DIR/
           python3 $NATVIS_TOOL_DIR/generate_natvis.py --version "$VERSION_MAJOR.$VERSION_MINOR.$VERSION_PATCH" $MAIN_DIR
 
       - name: Build patch and check for differences

tools/generate_natvis/nlohmann_json.natvis.j2

@@ -36,5 +36,31 @@
         </Expand>
     </Type>
 
+    <!-- Fallback for when the basic_json entry above does not match: json_default_base is the (empty) default
+         base class of basic_json, and base class visualizers are inherited by derived types and evaluated
+         against the derived object, so m_data is accessible here. The class lives in {{ ns }} after
+         3.12.0 (#5238) and in {{ ns }}::detail up to 3.12.0, so both names are listed. -->
+{% for default_base in ['json_default_base', 'detail::json_default_base'] %}
+    <Type Name="{{ ns }}::{{ default_base }}">
+        <DisplayString Condition="m_data.m_type == {{ ns }}::detail::value_t::null">null</DisplayString>
+        <DisplayString Condition="m_data.m_type == {{ ns }}::detail::value_t::object">{*(m_data.m_value.object)}</DisplayString>
+        <DisplayString Condition="m_data.m_type == {{ ns }}::detail::value_t::array">{*(m_data.m_value.array)}</DisplayString>
+        <DisplayString Condition="m_data.m_type == {{ ns }}::detail::value_t::string">{*(m_data.m_value.string)}</DisplayString>
+        <DisplayString Condition="m_data.m_type == {{ ns }}::detail::value_t::boolean">{m_data.m_value.boolean}</DisplayString>
+        <DisplayString Condition="m_data.m_type == {{ ns }}::detail::value_t::number_integer">{m_data.m_value.number_integer}</DisplayString>
+        <DisplayString Condition="m_data.m_type == {{ ns }}::detail::value_t::number_unsigned">{m_data.m_value.number_unsigned}</DisplayString>
+        <DisplayString Condition="m_data.m_type == {{ ns }}::detail::value_t::number_float">{m_data.m_value.number_float}</DisplayString>
+        <DisplayString Condition="m_data.m_type == {{ ns }}::detail::value_t::discarded">discarded</DisplayString>
+        <Expand>
+            <ExpandedItem Condition="m_data.m_type == {{ ns }}::detail::value_t::object">
+                *(m_data.m_value.object),view(simple)
+            </ExpandedItem>
+            <ExpandedItem Condition="m_data.m_type == {{ ns }}::detail::value_t::array">
+                *(m_data.m_value.array),view(simple)
+            </ExpandedItem>
+        </Expand>
+    </Type>
+
+{% endfor %}
 {% endfor %}
 </AutoVisualizer>