disallow `static const` variables without default-value (#7993)

* Fix static const variables without initializers causing internal errors

Add validation in SemanticsDeclHeaderVisitor::checkVarDeclCommon to detect static const variables without initializers and emit proper error diagnostics instead of allowing internal errors to escape during SPIR-V generation.

- Add new diagnostic (ID 31225) for static const variables without initializers
- Skip validation for extern static const variables
- Skip validation for interface member variables
- Add comprehensive test case covering various scenarios

Fixes #7989

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

* clean up test and implementation

* format code (#7994)

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

---------

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: ArielG-NV <ArielG-NV@users.noreply.github.com>
Co-authored-by: slangbot <ellieh+slangbot@nvidia.com>
Co-authored-by: slangbot <186143334+slangbot@users.noreply.github.com>

2 files changed, 37 insertions, 0 deletions

diff --git a/source/slang/slang-check-decl.cpp b/source/slang/slang-check-decl.cpp
index 4ebad6392..02f404d88 100644
--- a/source/slang/slang-check-decl.cpp
+++ b/source/slang/slang-check-decl.cpp
@@ -2356,6 +2356,37 @@ void SemanticsDeclHeaderVisitor::checkVarDeclCommon(VarDeclBase* varDecl)
         // global variables and struct fields to prevent mangling.
         addModifier(varDecl, m_astBuilder->create<ExternCppModifier>());
     }
+
+    // Check for static const variables without initializers
+    if (!varDecl->initExpr)
+    {
+        bool isStatic = false;
+        bool isConst = false;
+        bool isExtern = false;
+        for (auto modifier : varDecl->modifiers)
+        {
+            if (as<HLSLStaticModifier>(modifier))
+                isStatic = true;
+            else if (as<ConstModifier>(modifier))
+                isConst = true;
+            else if (as<ExternModifier>(modifier))
+                isExtern = true;
+
+            if (isStatic && isConst && isExtern)
+                break;
+        }
+        if (isStatic && isConst &&
+            // Don't error for extern variables
+            // Don't error for interface member variables
+            !isExtern && !as<InterfaceDecl>(varDecl->parentDecl))
+        {
+            getSink()->diagnose(
+                varDecl,
+                Diagnostics::staticConstVariableRequiresInitializer,
+                varDecl);
+        }
+    }
+
     checkVisibility(varDecl);
 }
 
diff --git a/source/slang/slang-diagnostic-defs.h b/source/slang/slang-diagnostic-defs.h
index 5544329a5..8f4845421 100644
--- a/source/slang/slang-diagnostic-defs.h
+++ b/source/slang/slang-diagnostic-defs.h
@@ -1510,6 +1510,12 @@ DIAGNOSTIC(
     constGlobalVarWithInitRequiresStatic,
     "global const variable with initializer must be declared static: '$0'")
 
+DIAGNOSTIC(
+    31225,
+    Error,
+    staticConstVariableRequiresInitializer,
+    "static const variable '$0' must have an initializer")
+
 // Enums
 
 DIAGNOSTIC(32000, Error, invalidEnumTagType, "invalid tag type for 'enum': '$0'")