Fix enum array indexing by adding implicit conversion support (#7815)

* Initial plan

* Fix enum array indexing by adding implicit conversion support

Co-authored-by: csyonghe <2652293+csyonghe@users.noreply.github.com>

* Update enum array indexing test to support GPU backends

Co-authored-by: csyonghe <2652293+csyonghe@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: csyonghe <2652293+csyonghe@users.noreply.github.com>

4 files changed, 55 insertions, 6 deletions

diff --git a/source/slang/slang-check-decl.cpp b/source/slang/slang-check-decl.cpp
index 1dc230dae..806912abe 100644
--- a/source/slang/slang-check-decl.cpp
+++ b/source/slang/slang-check-decl.cpp
@@ -8485,6 +8485,16 @@ bool SemanticsVisitor::isScalarIntegerType(Type* type)
     return isIntegerBaseType(baseType) || baseType == BaseType::Bool;
 }
 
+Type* SemanticsVisitor::getMatchingIntType(Type* type)
+{
+    if (isScalarIntegerType(type))
+        return type;
+    if (auto enumTypeDecl = isDeclRefTypeOf<EnumDecl>(type))
+        if (enumTypeDecl.getDecl()->tagType)
+            return getMatchingIntType(enumTypeDecl.getDecl()->tagType);
+    return m_astBuilder->getIntType();
+}
+
 bool SemanticsVisitor::isHalfType(Type* type)
 {
     auto basicType = as<BasicExpressionType>(type);
diff --git a/source/slang/slang-check-expr.cpp b/source/slang/slang-check-expr.cpp
index 41c3bd510..2891c316f 100644
--- a/source/slang/slang-check-expr.cpp
+++ b/source/slang/slang-check-expr.cpp
@@ -2490,13 +2490,10 @@ Expr* SemanticsVisitor::CheckSimpleSubscriptExpr(IndexExpr* subscriptExpr, Type*
     {
         expr = CheckExpr(expr);
     }
-    auto indexExpr = subscriptExpr->indexExprs[0];
+    auto& indexExpr = subscriptExpr->indexExprs[0];
 
-    if (!isScalarIntegerType(indexExpr->type.type))
-    {
-        getSink()->diagnose(indexExpr, Diagnostics::subscriptIndexNonInteger);
-        return CreateErrorExpr(subscriptExpr);
-    }
+    auto intTargetType = getMatchingIntType(indexExpr->type.type);
+    indexExpr = coerce(CoercionSite::Argument, intTargetType, indexExpr, getSink());
 
     subscriptExpr->type = QualType(elementType);
 
diff --git a/source/slang/slang-check-impl.h b/source/slang/slang-check-impl.h
index 86d0b42fe..5c6be2665 100644
--- a/source/slang/slang-check-impl.h
+++ b/source/slang/slang-check-impl.h
@@ -2139,6 +2139,12 @@ public:
     /// Is `type` a scalar integer type.
     bool isScalarIntegerType(Type* type);
 
+    // This function is used to get the best integer type that matches the given type.
+    // If `type` is already an integer type, return it as is.
+    // If `type` is a enum type, return the tag type if it exists.
+    // Otherwise, return the 32-bit signed integer type.
+    Type* getMatchingIntType(Type* type);
+
     /// Is `type` a scalar half type.
     bool isHalfType(Type* type);
 
diff --git a/tests/language-feature/enums/enum-array-indexing.slang b/tests/language-feature/enums/enum-array-indexing.slang
new file mode 100644
index 000000000..c9294ad0c
--- /dev/null
+++ b/tests/language-feature/enums/enum-array-indexing.slang
@@ -0,0 +1,36 @@
+//TEST(compute):COMPARE_COMPUTE(filecheck-buffer=CHECK): -shaderobj -output-using-type
+//TEST(compute):COMPARE_COMPUTE(filecheck-buffer=CHECK):-cpu -shaderobj -output-using-type
+//TEST(compute, vulkan):COMPARE_COMPUTE(filecheck-buffer=CHECK):-vk -shaderobj -output-using-type
+
+// Test that enums can be used as array indices without explicit casting
+
+enum Fruit { Apple, Orange, Banana };
+
+//TEST_INPUT: set outputBuffer = out ubuffer(data=[0 0 0 0], stride=4)
+RWStructuredBuffer<int> outputBuffer;
+
+[numthreads(1,1,1)]
+void computeMain()
+{
+    int fruits[10];
+    
+    // Initialize arrays with some values
+    for (int i = 0; i < 10; i++)
+    {
+        fruits[i] = i * 10;
+    }
+    
+    // Test basic enum indexing - this should work with our fix
+    int appleCost = fruits[Fruit::Apple];     // Should access fruits[0] = 0
+    int orangeCost = fruits[Fruit::Orange];   // Should access fruits[1] = 10
+    int bananaCost = fruits[Fruit::Banana];   // Should access fruits[2] = 20
+    
+    // CHECK: 0
+    outputBuffer[0] = appleCost;
+    // CHECK: 10
+    outputBuffer[1] = orangeCost;
+    // CHECK: 20
+    outputBuffer[2] = bananaCost;
+    // CHECK: 42
+    outputBuffer[3] = 42; // Just a test value
+}
\ No newline at end of file