Fix zero size array handling in slangc (#6399)

* Fix zero size array handling in slangc

Fixes #2890

1. Fix zero size array handling in slangc
2. Add new zero size array diagnostic test.

* format code

* fix review comments

---------

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

Author: 3 people

source/slang/slang-check-expr.cpp

@@ -2378,6 +2378,18 @@ Expr* SemanticsExprVisitor::visitIndexExpr(IndexExpr* subscriptExpr)
                 IntegerConstantExpressionCoercionType::AnyInteger,
                 nullptr,
                 ConstantFoldingKind::LinkTime);
+
+            // Validate that array size is greater than zero
+            if (auto constElementCount = as<ConstantIntVal>(elementCount))
+            {
+                if (constElementCount->getValue() <= 0)
+                {
+                    getSink()->diagnose(
+                        subscriptExpr->indexExprs[0],
+                        Diagnostics::invalidArraySize);
+                    return CreateErrorExpr(subscriptExpr);
+                }
+            }
         }
         else if (subscriptExpr->indexExprs.getCount() != 0)
         {

tests/diagnostics/array-zero-size.slang

@@ -2,12 +2,12 @@
 
 // Test that array size cannot be zero
 
-//TEST:SIMPLE:
+//DIAGNOSTIC_TEST:SIMPLE(filecheck=CHECK):
 
 [numthreads(4, 1, 1)]
 void computeMain(uint3 dispatchThreadID : SV_DispatchThreadID)
 {
     bar();
 }
-
-func bar() -> int[0];  // expected-error 30025 "array size must be larger than zero." 
+//CHECK: ([[# @LINE+1]]): error 30025
+func bar() -> int[0];