From 3fbebee466ce70a37bca66e26eb785976aaa538b Mon Sep 17 00:00:00 2001
From: Don Yin <Don_Yin@outlook.com>
Date: Mon, 26 Oct 2026 16:06:58 +0000
Subject: [PATCH 1/2] Guard cfg'd fields in generated C++ operator== and
 operator!=

Fixes #937

Signed-off-by: Don Yin <Don_Yin@outlook.com>
---
 src/bindgen/language_backend/clike.rs | 43 +++++++++++++++++++--------
 tests/expectations/cfg.cpp            | 15 ++++++++++
 tests/rust/cfg.rs                     |  3 --
 3 files changed, 45 insertions(+), 16 deletions(-)

diff --git a/src/bindgen/language_backend/clike.rs b/src/bindgen/language_backend/clike.rs
index 4d927abe2..c105bc54d 100644
--- a/src/bindgen/language_backend/clike.rs
+++ b/src/bindgen/language_backend/clike.rs
@@ -287,19 +287,36 @@ impl<'a> CLikeLanguageBackend<'a> {
                     other
                 );
                 out.open_brace();
-                out.write("return ");
-                let vec: Vec<_> = s
-                    .fields
-                    .iter()
-                    .skip(skip_fields)
-                    .map(|field| format!("{} {} {}.{}", field.name, $op, other, field.name))
-                    .collect();
-                out.write_vertical_source_list(
-                    self,
-                    &vec[..],
-                    ListType::Join(&format!(" {}", $conjuc)),
-                    |_, out, s| write!(out, "{}", s),
-                );
+                out.write("return");
+                let fields = &s.fields[skip_fields..];
+                if matches!($op, "==" | "!=") && fields.iter().any(|field| field.cfg.is_some()) {
+                    out.new_line();
+                    for field in fields {
+                        let condition = field.cfg.to_condition(self.config);
+                        condition.write_before(self.config, out);
+                        write!(
+                            out,
+                            "{} {} {}.{} {}",
+                            field.name, $op, other, field.name, $conjuc
+                        );
+                        condition.write_after(self.config, out);
+                        out.new_line();
+                    }
+                    // A neutral operand keeps the expression valid even if all fields are absent.
+                    write!(out, "{}", $op == "==");
+                } else {
+                    out.write(" ");
+                    let vec: Vec<_> = fields
+                        .iter()
+                        .map(|field| format!("{} {} {}.{}", field.name, $op, other, field.name))
+                        .collect();
+                    out.write_vertical_source_list(
+                        self,
+                        &vec[..],
+                        ListType::Join(&format!(" {}", $conjuc)),
+                        |_, out, s| write!(out, "{}", s),
+                    );
+                }
                 out.write(";");
                 out.close_brace(false);
             }};
diff --git a/tests/expectations/cfg.cpp b/tests/expectations/cfg.cpp
index 13063484b..052876311 100644
--- a/tests/expectations/cfg.cpp
+++ b/tests/expectations/cfg.cpp
@@ -266,6 +266,21 @@ struct ConditionalField {
   int32_t field
 #endif
   ;
+
+  bool operator==(const ConditionalField& other) const {
+    return
+#if defined(X11)
+    field == other.field &&
+#endif
+    true;
+  }
+  bool operator!=(const ConditionalField& other) const {
+    return
+#if defined(X11)
+    field != other.field ||
+#endif
+    false;
+  }
 };
 constexpr static const ConditionalField ConditionalField_ZERO = ConditionalField{
 #if defined(X11)
diff --git a/tests/rust/cfg.rs b/tests/rust/cfg.rs
index b42c1d1a7..a002333d2 100644
--- a/tests/rust/cfg.rs
+++ b/tests/rust/cfg.rs
@@ -60,9 +60,6 @@ struct BarHandle {
     y: f32,
 }
 
-// FIXME(#634): Support deriving methods for structs with conditional fields.
-/// cbindgen:derive-eq=false
-/// cbindgen:derive-neq=false
 #[repr(C)]
 struct ConditionalField {
     #[cfg(x11)]

From 002c66a61ec7fcbdd9a458de536df1f21beb1c93 Mon Sep 17 00:00:00 2001
From: Don Yin <Don_Yin@outlook.com>
Date: Mon, 26 Oct 2026 16:08:01 +0000
Subject: [PATCH 2/2] Add a CHANGES entry

Signed-off-by: Don Yin <Don_Yin@outlook.com>
---
 CHANGES | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/CHANGES b/CHANGES
index 6fd28678c..f34ae9eda 100644
--- a/CHANGES
+++ b/CHANGES
@@ -1,5 +1,8 @@
 # unreleased
 
+      * Wrap conditional fields in `#if` blocks in generated C++ `operator==` and
+        `operator!=`, so headers still compile when a feature is disabled (#937).
+
       * Emit `&str`, `&CStr`, and arrays thereof as C string literals, and byte-string
         (`b"..."`) constants as `uint8_t[]` arrays, instead of dropping them.