From cdaeccd7d460697b2db9785f55e03eaabdb8b9de Mon Sep 17 00:00:00 2001
From: Rainer <r.sagitario@gmx.de>
Date: Sun, 14 Jun 2026 10:01:53 +0200
Subject: [PATCH 1/2] fix #23262 - [REG v2.113.0-beta.1] ICE when trying to
 cast a class instance to a C++ interface

no need to exclude C++ classes/interfaces from runtime cast target
---
 compiler/src/dmd/expressionsem.d |  2 +-
 compiler/test/runnable/casting.d | 22 ++++++++++++++++++++++
 2 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/compiler/src/dmd/expressionsem.d b/compiler/src/dmd/expressionsem.d
index 0ccd016b9c2d..7474b607b52e 100644
--- a/compiler/src/dmd/expressionsem.d
+++ b/compiler/src/dmd/expressionsem.d
@@ -4994,7 +4994,7 @@ private void lowerCastExp(CastExp cex, Scope* sc)
 
     int offset;
     if ((cdto.isBaseOf(cdfrom, &offset) && offset != ClassDeclaration.OFFSET_RUNTIME)
-        || cdfrom.classKind == ClassKind.cpp || cdto.classKind == ClassKind.cpp)
+        || cdfrom.classKind == ClassKind.cpp)
         return;
 
     Identifier hook = Id._d_cast;
diff --git a/compiler/test/runnable/casting.d b/compiler/test/runnable/casting.d
index 1eb262c5c682..9934fe50fa38 100644
--- a/compiler/test/runnable/casting.d
+++ b/compiler/test/runnable/casting.d
@@ -210,6 +210,27 @@ void test14218()
     }
 }
 
+/***************************************************/
+// https://github.com/dlang/dmd/issues/23262
+
+extern(C++) interface iface23262
+{
+    int funCpp();
+}
+
+class class23262 : Object, iface23262
+{
+    extern(C++)	int funCpp() { return 42; }
+}
+
+void test23262()
+{
+    Object obj = new class23262;
+    auto cpp = cast(iface23262) obj;
+    assert(cpp);
+    assert(cpp.funCpp() == 42);
+}
+
 /***************************************************/
 
 int main()
@@ -223,6 +244,7 @@ int main()
     test10842();
     test11722();
     test14218();
+    test23262();
 
     printf("Success\n");
     return 0;

From 193a6edbdee27354b057f9349805a5c954eede30 Mon Sep 17 00:00:00 2001
From: Rainer <r.sagitario@gmx.de>
Date: Thu, 18 Jun 2026 21:58:46 +0200
Subject: [PATCH 2/2] generate null lowering for mismatching linkage types

---
 compiler/src/dmd/expressionsem.d | 17 +++++++++++++++--
 compiler/test/runnable/casting.d | 17 ++++++++++++++++-
 2 files changed, 31 insertions(+), 3 deletions(-)

diff --git a/compiler/src/dmd/expressionsem.d b/compiler/src/dmd/expressionsem.d
index 7474b607b52e..526faaaeccbe 100644
--- a/compiler/src/dmd/expressionsem.d
+++ b/compiler/src/dmd/expressionsem.d
@@ -4993,10 +4993,23 @@ private void lowerCastExp(CastExp cex, Scope* sc)
     ClassDeclaration cdto   = tob.isClassHandle();
 
     int offset;
-    if ((cdto.isBaseOf(cdfrom, &offset) && offset != ClassDeclaration.OFFSET_RUNTIME)
-        || cdfrom.classKind == ClassKind.cpp)
+    if (cdto.isBaseOf(cdfrom, &offset) && offset != ClassDeclaration.OFFSET_RUNTIME)
+        return; // codegen has to deal with pointer adjustment
+
+    if (cdfrom.classKind != ClassKind.d ||
+        (cdto.classKind != ClassKind.d &&
+         !(cdto.classKind == ClassKind.cpp && cdto.isInterfaceDeclaration())))
+    {
+        if (cdfrom.classKind == cdto.classKind)
+            return; // for non-D classes, generate a reinterpreting cast
+
+        // conversion other than D -> C++ interface result in null
+        Expression lowerNull = new NullExp(cex.loc, cex.to);
+        cex.lowering = lowerNull .expressionSemantic(sc);
         return;
+    }
 
+    // cast D -> D / C++ interface calls _d_cast
     Identifier hook = Id._d_cast;
     if (!verifyHookExist(cex.loc, *sc, hook, "d_cast", Id.object))
         return;
diff --git a/compiler/test/runnable/casting.d b/compiler/test/runnable/casting.d
index 9934fe50fa38..25f7a63e03fd 100644
--- a/compiler/test/runnable/casting.d
+++ b/compiler/test/runnable/casting.d
@@ -218,6 +218,11 @@ extern(C++) interface iface23262
     int funCpp();
 }
 
+extern(C++) class cpp23262
+{
+    int funCpp() { return 1; }
+}
+
 class class23262 : Object, iface23262
 {
     extern(C++)	int funCpp() { return 42; }
@@ -226,9 +231,19 @@ class class23262 : Object, iface23262
 void test23262()
 {
     Object obj = new class23262;
-    auto cpp = cast(iface23262) obj;
+    auto cpp = cast(iface23262) obj; // ok for C++ interface
     assert(cpp);
     assert(cpp.funCpp() == 42);
+
+    auto cpp2 = cast(cpp23262) obj;
+    assert(cpp2 is null); // impossible for C++ class
+
+    auto d = cast(class23262) cpp;
+    assert(d is null); // no way back
+
+    auto cppobj = new cpp23262;
+    auto d2 = cast(class23262) cppobj;
+    assert(d2 is null); // classes of different linkage never mix
 }
 
 /***************************************************/