From b46844a30b751e6abd5eef59d60f99d5ca3844aa Mon Sep 17 00:00:00 2001
From: "Harry T. Vennik" <htvennik@gmail.com>
Date: Fri, 12 Feb 2021 13:38:24 +0100
Subject: [PATCH] Fix Issue 21631 - cas: add 'const' attribute to local
 variable 'arg1'

---
 src/core/atomic.d | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/src/core/atomic.d b/src/core/atomic.d
index e6a82e58f8..4af3fdf2bd 100644
--- a/src/core/atomic.d
+++ b/src/core/atomic.d
@@ -292,7 +292,7 @@ template cas(MemoryOrder succ = MemoryOrder.seq, MemoryOrder fail = MemoryOrder.
     in (atomicPtrIsProperlyAligned(here), "Argument `here` is not properly aligned")
     {
         // resolve implicit conversions
-        T arg1 = ifThis;
+        const T arg1 = ifThis;
         T arg2 = writeThis;
 
         static if (__traits(isFloating, T))
@@ -1276,4 +1276,13 @@ version (CoreUnittest)
         shared NoIndirections n;
         static assert(is(typeof(atomicLoad(n)) == NoIndirections));
     }
+
+    unittest // Issue 21631
+    {
+        shared uint si1 = 45;
+        shared uint si2 = 38;
+        shared uint* psi = &si1;
+
+        assert((&psi).cas(cast(const) psi, &si2));
+    }
 }