From 30d8ddd65aba303cf746ca3b06bf59d01349e392 Mon Sep 17 00:00:00 2001
From: Niels Lohmann <mail@nlohmann.me>
Date: Fri, 9 Oct 2026 23:48:40 +0200
Subject: [PATCH] Fix MSVC C4127 in the lookups of editable json documents

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
---
 include/nlohmann/detail/view/document_data.hpp | 12 ++++++++++++
 include/nlohmann/detail/view/lookup.hpp        |  7 ++++---
 2 files changed, 16 insertions(+), 3 deletions(-)

diff --git a/include/nlohmann/detail/view/document_data.hpp b/include/nlohmann/detail/view/document_data.hpp
index 2e1eac3ef..dd2c6be75 100644
--- a/include/nlohmann/detail/view/document_data.hpp
+++ b/include/nlohmann/detail/view/document_data.hpp
@@ -217,6 +217,13 @@ struct document_data
 template<bool Editable>
 struct navigation
 {
+    /// whether the walk follows edits (the template argument as a runtime
+    /// condition: `Editable && ...` is a constant condition for MSVC, C4127)
+    static NLOHMANN_VIEW_ALWAYS_INLINE bool editable() noexcept
+    {
+        return false;
+    }
+
     static NLOHMANN_VIEW_ALWAYS_INLINE const node* first(const document_data& /*d*/, const node* n) noexcept
     {
         return n + 1;
@@ -236,6 +243,11 @@ struct navigation
 template<>
 struct navigation<true>
 {
+    static NLOHMANN_VIEW_ALWAYS_INLINE bool editable() noexcept
+    {
+        return true;
+    }
+
     static NLOHMANN_VIEW_ALWAYS_INLINE const node* first(const document_data& d, const node* n) noexcept
     {
         return d.first_child_edited(n);
diff --git a/include/nlohmann/detail/view/lookup.hpp b/include/nlohmann/detail/view/lookup.hpp
index afc43e95a..5bfe36b94 100644
--- a/include/nlohmann/detail/view/lookup.hpp
+++ b/include/nlohmann/detail/view/lookup.hpp
@@ -92,7 +92,7 @@ template<bool Editable>
 const node* find_member(const document_data& d, const node* object, const char* key, std::size_t n) noexcept
 {
     using nav = navigation<Editable>;
-    if (NLOHMANN_VIEW_UNLIKELY(object->extra != 0) && (!Editable || (object->flags & node_flags::moved) == 0))
+    if (NLOHMANN_VIEW_UNLIKELY(object->extra != 0) && (!nav::editable() || (object->flags & node_flags::moved) == 0))
     {
         return find_indexed(d, object, key, n); // a large object (whose members have not been edited)
     }
@@ -143,8 +143,9 @@ SizeType to_index(IntegerType idx) noexcept
 template<bool Editable>
 const node* element_at(const document_data& d, const node* array, std::size_t idx) noexcept
 {
-    const node* e = navigation<Editable>::first(d, array);
-    if (Editable && (array->flags & node_flags::moved) != 0 && d.edits->moved_cap[array->off] != 0)
+    using nav = navigation<Editable>;
+    const node* e = nav::first(d, array);
+    if (nav::editable() && (array->flags & node_flags::moved) != 0 && d.edits->moved_cap[array->off] != 0)
     {
         return e + idx; // a growable block: one link per element
     }