From 79ae21611861656b58dbb0b75c82408c4db47a57 Mon Sep 17 00:00:00 2001
From: Divyansh Sharma <divyanshsharma4507@gmail.com>
Date: Tue, 3 Mar 2026 22:24:21 +0530
Subject: [PATCH] Fix issue 22175 - require ref for void AA update callbacks

---
 compiler/test/fail_compilation/fail22175.d | 18 ++++++++++++++++++
 druntime/src/object.d                      | 22 ++++++++++++++++++----
 2 files changed, 36 insertions(+), 4 deletions(-)
 create mode 100644 compiler/test/fail_compilation/fail22175.d

diff --git a/compiler/test/fail_compilation/fail22175.d b/compiler/test/fail_compilation/fail22175.d
new file mode 100644
index 000000000000..5c393edaddf7
--- /dev/null
+++ b/compiler/test/fail_compilation/fail22175.d
@@ -0,0 +1,18 @@
+// https://github.com/dlang/dmd/issues/22175
+// void-returning update callback for AA must take value by ref
+/*
+TEST_OUTPUT:
+---
+fail_compilation/fail22175.d(14): Error: static assert:  "void-returning update callback must take ref parameter"
+---
+*/
+
+void main()
+{
+    int[int] aa;
+    // Regression guard: this callback shape must never compile.
+    static assert(
+        is(typeof(aa.update(1, () => 10, delegate void(int x) { x += 1; }))),
+        "void-returning update callback must take ref parameter"
+    );
+}
diff --git a/druntime/src/object.d b/druntime/src/object.d
index bb0c3d3c2738..47bd85b8c67a 100644
--- a/druntime/src/object.d
+++ b/druntime/src/object.d
@@ -3544,10 +3544,12 @@ private enum bool isSafeCopyable(T) = is(typeof(() @safe { union U { T x; } T *x
  *      create = The callable to create a value for `key`.
  *               Must return V.
  *      update = The callable to call if `key` exists.
- *               Takes a V argument, returns a V or void.
+ *               Takes a ref V or V argument. Returns a V or void.
+ *               If it returns void, the parameter must be ref.
  */
 void update(K, V, C, U)(ref V[K] aa, K key, scope C create, scope U update)
-if (is(typeof(create()) : V) && (is(typeof(update(aa[K.init])) : V) || is(typeof(update(aa[K.init])) == void)))
+if (is(typeof(create()) : V) && (is(typeof(update(aa[K.init])) : V)
+    || (is(typeof(update(aa[K.init])) == void) && !is(typeof(update(V.init)) == void))))
 {
     bool found;
     auto p = _aaGetX(aa, key, found, create());
@@ -3568,7 +3570,7 @@ if (is(typeof(create()) : V) && (is(typeof(update(aa[K.init])) : V) || is(typeof
     // create
     aa.update("key",
         () => 1,
-        (int) {} // not executed
+        (ref int) {} // not executed
         );
     assert(aa["key"] == 1);
 
@@ -3590,7 +3592,7 @@ if (is(typeof(create()) : V) && (is(typeof(update(aa[K.init])) : V) || is(typeof
     // 'update' without changing value
     aa.update("key",
         () => 0, // not executed
-        (int) {
+        (ref int) {
             // do something else
         });
     assert(aa["key"] == 4);
@@ -3626,6 +3628,18 @@ if (is(typeof(create()) : V) && (is(typeof(update(aa[K.init])) : V) || is(typeof
     static assert(!is(typeof(() @safe { aais.update(S(1234), { return 1234; }, (ref int x) { x++; return x; }); })));
 }
 
+// void-returning update callback must take parameter by ref (issue 22175)
+@safe unittest
+{
+    int[string] aa;
+    // void-returning by-value must be rejected
+    static assert(!is(typeof(() { aa.update("key", () => 0, (int v) { }); })));
+    // void-returning by-ref must be accepted
+    static assert(is(typeof(() { aa.update("key", () => 0, (ref int v) { }); })));
+    // non-void returning by-value is fine (value used to update)
+    static assert(is(typeof(() { aa.update("key", () => 0, (int v) => v * 2); })));
+}
+
 @safe unittest
 {
     struct S0