Diagnose on use of struct inheritance. (#7419)

* Diagnose on use of struct inheritance.

* fix test.

* Fix tests.

* fix.

---------

Co-authored-by: ArielG-NV <159081215+ArielG-NV@users.noreply.github.com>

2 files changed, 30 insertions, 5 deletions

diff --git a/source/slang/slang-check-decl.cpp b/source/slang/slang-check-decl.cpp
index 2865188b4..ddc4ec4d5 100644
--- a/source/slang/slang-check-decl.cpp
+++ b/source/slang/slang-check-decl.cpp
@@ -7811,7 +7811,7 @@ void SemanticsDeclBasesVisitor::visitStructDecl(StructDecl* decl)
         {
             getSink()->diagnose(
                 inheritanceDecl,
-                Diagnostics::baseOfStructMustBeStructOrInterface,
+                Diagnostics::baseOfStructMustBeInterface,
                 decl,
                 baseType);
             continue;
@@ -7823,6 +7823,26 @@ void SemanticsDeclBasesVisitor::visitStructDecl(StructDecl* decl)
         }
         else if (auto baseStructDeclRef = baseDeclRef.as<StructDecl>())
         {
+            if (!isFromCoreModule(decl))
+            {
+                // In Slang 2026, we no longer allow structs to inherit from other structs.
+                if (isSlang2026OrLater(this))
+                {
+                    getSink()->diagnose(
+                        inheritanceDecl,
+                        Diagnostics::baseOfStructMustBeInterface,
+                        decl,
+                        baseType);
+                }
+                else
+                {
+                    // For legacy langauge versions, we still allow struct inheritance to avoid
+                    // breaking existing code, but we will emit a warning to inform the user
+                    // that this feature is unstable and may be removed in the future.
+                    getSink()->diagnose(inheritanceDecl, Diagnostics::inheritanceUnstable);
+                }
+            }
+
             // To simplify the task of reading and maintaining code,
             // we require that when a `struct` inherits from another
             // `struct`, the base `struct` is the first item in
@@ -7845,7 +7865,7 @@ void SemanticsDeclBasesVisitor::visitStructDecl(StructDecl* decl)
         {
             getSink()->diagnose(
                 inheritanceDecl,
-                Diagnostics::baseOfStructMustBeStructOrInterface,
+                Diagnostics::baseOfStructMustBeInterface,
                 decl,
                 baseType);
             continue;
diff --git a/source/slang/slang-diagnostic-defs.h b/source/slang/slang-diagnostic-defs.h
index f4741ab04..4f0edf53b 100644
--- a/source/slang/slang-diagnostic-defs.h
+++ b/source/slang/slang-diagnostic-defs.h
@@ -1662,8 +1662,8 @@ DIAGNOSTIC(
 DIAGNOSTIC(
     30811,
     Error,
-    baseOfStructMustBeStructOrInterface,
-    "struct '$0' cannot inherit from type '$1' that is neither a struct nor an interface")
+    baseOfStructMustBeInterface,
+    "struct '$0' cannot inherit from non-interface type '$1'")
 DIAGNOSTIC(
     30812,
     Error,
@@ -1681,7 +1681,12 @@ DIAGNOSTIC(
     baseOfClassMustBeClassOrInterface,
     "class '$0' cannot inherit from type '$1' that is neither a class nor an interface")
 DIAGNOSTIC(30815, Error, circularityInExtension, "circular extension is not allowed.")
-
+DIAGNOSTIC(
+    30816,
+    Warning,
+    inheritanceUnstable,
+    "support for inheritance is unstable and will be removed in future language versions, consider "
+    "using composition instead.")
 DIAGNOSTIC(
     30820,
     Error,