From b01ae659139e62f54d4e869dd0e1ccbe7ca0326f Mon Sep 17 00:00:00 2001
From: Abul <abulkhan19175@gmail.com>
Date: Tue, 28 Jul 2026 12:14:59 +0530
Subject: [PATCH] fix

---
 compiler/src/dmd/funcsem.d                  |  7 ++++-
 compiler/test/fail_compilation/issue23470.d | 31 +++++++++++++++++++++
 2 files changed, 37 insertions(+), 1 deletion(-)
 create mode 100644 compiler/test/fail_compilation/issue23470.d

diff --git a/compiler/src/dmd/funcsem.d b/compiler/src/dmd/funcsem.d
index 657cb2f085d1..e38fe89e6857 100644
--- a/compiler/src/dmd/funcsem.d
+++ b/compiler/src/dmd/funcsem.d
@@ -826,10 +826,15 @@ void funcDeclarationSemantic(Scope* sc, FuncDeclaration funcdecl)
     if (auto id = parent.isInterfaceDeclaration())
     {
         funcdecl.storage_class |= STC.abstract_;
-        if (funcdecl.isCtorDeclaration() || funcdecl.isPostBlitDeclaration() || funcdecl.isDtorDeclaration() || funcdecl.isInvariantDeclaration() || funcdecl.isNewDeclaration() || funcdecl.isDelete())
+        const isSpecialFunc = funcdecl.isCtorDeclaration() || funcdecl.isPostBlitDeclaration() || funcdecl.isDtorDeclaration() || funcdecl.isInvariantDeclaration() || funcdecl.isNewDeclaration() || funcdecl.isDelete();
+        if (isSpecialFunc)
             .error(funcdecl.loc, "%s `%s` constructors, destructors, postblits, invariants, new and delete functions are not allowed in interface `%s`", funcdecl.kind, funcdecl.toPrettyChars, id.toErrMsg());
         if (funcdecl.fbody && funcdecl.isVirtual())
             .error(funcdecl.loc, "%s `%s` function body only allowed in `final` functions in interface `%s`", funcdecl.kind, funcdecl.toPrettyChars, id.toErrMsg());
+        // A final interface function has no vtable slot, so no class can supply its
+        // body. Only a foreign linkage can, from the other side.
+        if (!isSpecialFunc && !funcdecl.fbody && !funcdecl.isStatic() && funcdecl.isFinalFunc() && funcdecl.resolvedLinkage() == LINK.d)
+            .error(funcdecl.loc, "%s `%s` `final` function requires a body in interface `%s`", funcdecl.kind, funcdecl.toPrettyChars, id.toErrMsg());
     }
 
     if (UnionDeclaration ud = parent.isUnionDeclaration())
diff --git a/compiler/test/fail_compilation/issue23470.d b/compiler/test/fail_compilation/issue23470.d
new file mode 100644
index 000000000000..2f8e500ebde1
--- /dev/null
+++ b/compiler/test/fail_compilation/issue23470.d
@@ -0,0 +1,31 @@
+/*
+TEST_OUTPUT:
+---
+fail_compilation/issue23470.d(14): Error: function `issue23470.A.foo` `final` function requires a body in interface `A`
+---
+*/
+
+// https://github.com/dlang/dmd/issues/23470
+
+final:
+
+interface A
+{
+    void foo();
+}
+
+class B : A
+{
+    void foo() { }
+}
+
+// exempt, the definition can come from the other language
+extern (C++) interface C
+{
+    void bar();
+}
+
+extern (C) interface D
+{
+    void baz();
+}