From 1b34fea4788136b54ec77c6ed9678754d109fc79 Mon Sep 17 00:00:00 2001
From: Dennis <dkorpel@users.noreply.github.com>
Date: Thu, 10 Apr 2025 23:05:48 +0200
Subject: [PATCH] Fix #21179 - Failure to convert const(T) to T after type is
 used in cast() (#21201)

---
 compiler/src/dmd/typesem.d           | 16 +++++++++++++---
 compiler/test/compilable/test21179.d | 11 +++++++++++
 2 files changed, 24 insertions(+), 3 deletions(-)
 create mode 100644 compiler/test/compilable/test21179.d

diff --git a/compiler/src/dmd/typesem.d b/compiler/src/dmd/typesem.d
index 3bc0489bff..d4c7a5865d 100644
--- a/compiler/src/dmd/typesem.d
+++ b/compiler/src/dmd/typesem.d
@@ -3266,9 +3266,19 @@ Type merge(Type type)
 
         case Tsarray:
             // prevents generating the mangle if the array dim is not yet known
-            if (!type.isTypeSArray().dim.isIntegerExp())
-                return type;
-            goto default;
+            if (auto ie = type.isTypeSArray().dim.isIntegerExp())
+            {
+                // After TypeSemantic, the length is always converted to size_t, but the parser
+                // usually generates regular integer types (e.g. in cast(const ubyte[2])) which
+                // it may try to merge, which then leads to failing implicit conversions as 2LU != 2
+                // according to Expression.equals. Only merge array types with size_t lengths for now.
+                // https://github.com/dlang/dmd/issues/21179
+                if (ie.type != Type.tsize_t)
+                    return type;
+
+                goto default;
+            }
+            return type;
 
         case Tenum:
             break;
diff --git a/compiler/test/compilable/test21179.d b/compiler/test/compilable/test21179.d
new file mode 100644
index 0000000000..78bdffda55
--- /dev/null
+++ b/compiler/test/compilable/test21179.d
@@ -0,0 +1,11 @@
+// https://github.com/dlang/dmd/issues/21179
+
+void bigEndianToNative(ubyte[2] a) {}
+
+void main()
+{
+    ubyte[] arr;
+    const ubyte[2] bytes;
+    bigEndianToNative(bytes);
+    auto b = cast(const ubyte[2][]) arr;
+}