Ensure we do not have an `initExpr` on a `VarDecl` inside an `InterfaceDecl` (#7283)

* Ensure we do not have an initExpr on a var inside an InterfaceDecl

Ensure we do not have an initExpr on a var inside an InterfaceDecl. If we do, send an error.
Ensure the language server does not segfault with this error as per the issue.

* format code

* split tests

---------

Co-authored-by: slangbot <186143334+slangbot@users.noreply.github.com>

3 files changed, 41 insertions, 0 deletions

diff --git a/source/slang/slang-check-decl.cpp b/source/slang/slang-check-decl.cpp
index 71aa71e69..cc65edd2f 100644
--- a/source/slang/slang-check-decl.cpp
+++ b/source/slang/slang-check-decl.cpp
@@ -2464,6 +2464,17 @@ bool DiagnoseIsAllowedInitExpr(VarDeclBase* varDecl, DiagnosticSink* sink)
         return false;
     }
 
+    if (as<InterfaceDecl>(varDecl->parentDecl))
+    {
+        if (sink && varDecl->initExpr)
+            sink->diagnose(
+                varDecl,
+                Diagnostics::cannotHaveInitializer,
+                varDecl,
+                "an interface requirement");
+        return false;
+    }
+
     return true;
 }
 
diff --git a/tests/bugs/static-const-init-expr-with-interface-1.slang b/tests/bugs/static-const-init-expr-with-interface-1.slang
new file mode 100644
index 000000000..bab63c167
--- /dev/null
+++ b/tests/bugs/static-const-init-expr-with-interface-1.slang
@@ -0,0 +1,15 @@
+//TEST:LANG_SERVER(filecheck=CHECK):
+
+//HOVER:7,25
+//CHECK:Bug.badVar = 0
+interface Bug
+{
+    static const int badVar = 0;
+}
+
+RWStructuredBuffer<int> b;
+[numthreads(1, 1, 1)]
+void computeMain(uint3 dispatchThreadID: SV_DispatchThreadID)
+{
+    b[0] = Bug::badVar;
+}
diff --git a/tests/bugs/static-const-init-expr-with-interface-2.slang b/tests/bugs/static-const-init-expr-with-interface-2.slang
new file mode 100644
index 000000000..6976f1303
--- /dev/null
+++ b/tests/bugs/static-const-init-expr-with-interface-2.slang
@@ -0,0 +1,15 @@
+//TEST:SIMPLE(filecheck=CHECK): -stage compute -entry computeMain -target spirv
+
+//CHECK: error 30623
+
+interface Bug
+{
+    static const int badVar = 0;
+}
+
+RWStructuredBuffer<int> b;
+[numthreads(1, 1, 1)]
+void computeMain(uint3 dispatchThreadID: SV_DispatchThreadID)
+{
+    b[0] = Bug::badVar;
+}