[SYCL] add clang diagnostic for kernel free function (#18329)

1. Kernel free function should not support arguments with default values
according to
[docs](https://github.com/intel/llvm/blob/sycl/sycl/doc/extensions/proposed/sycl_ext_oneapi_free_function_kernels.asciidoc#defining-a-free-function-kernel).
2. Kernel free function should have return type void only, diagnostic
message was added to clarify that.
PR adds diagnostic messages for this cases.

Author: dklochkov-emb

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -12670,8 +12670,13 @@ def err_registered_kernels_name_already_registered : Error<
   "free function kernel has already been registered with '%0'; cannot register with '%1'">;
 def err_not_sycl_free_function : Error<
   "attempting to register a function that is not a SYCL free function as '%0'">;
-def err_free_function_variadic_args: Error<
+def err_free_function_with_default_arg : Error<
+  "a function with a default argument value cannot be used to define SYCL free function kernel">;
+def err_free_function_variadic_args : Error<
   "free function kernel cannot be a variadic function">;
+def err_free_function_return_type : Error<
+  "SYCL free function kernel should have return type 'void'">;
+
 
 // SYCL kernel entry point diagnostics
 def err_sycl_entry_point_invalid : Error<

clang/lib/Sema/SemaSYCL.cpp

@@ -1159,21 +1159,13 @@ static target getAccessTarget(QualType FieldTy,
       AccTy->getTemplateArgs()[3].getAsIntegral().getExtValue());
 }
 
-// FIXME: Free functions must have void return type and be declared at file
-// scope, outside any namespaces.
 bool SemaSYCL::isFreeFunction(const FunctionDecl *FD) {
   for (auto *IRAttr : FD->specific_attrs<SYCLAddIRAttributesFunctionAttr>()) {
     SmallVector<std::pair<std::string, std::string>, 4> NameValuePairs =
         IRAttr->getAttributeNameValuePairs(getASTContext());
     for (const auto &NameValuePair : NameValuePairs) {
       if (NameValuePair.first == "sycl-nd-range-kernel" ||
           NameValuePair.first == "sycl-single-task-kernel") {
-        if (!FD->getReturnType()->isVoidType()) {
-          llvm::report_fatal_error(
-              "Only functions at file scope with void return "
-              "type are permitted as free functions");
-          return false;
-        }
         return true;
       }
     }
@@ -5791,12 +5783,29 @@ void SemaSYCL::MarkDevices() {
   }
 }
 
+static bool CheckFreeFunctionDiagnostics(Sema &S, FunctionDecl *FD) {
+  if (FD->isVariadic()) {
+    return S.Diag(FD->getLocation(), diag::err_free_function_variadic_args);
+  }
+
+  if (!FD->getReturnType()->isVoidType()) {
+    return S.Diag(FD->getLocation(), diag::err_free_function_return_type);
+  }
+
+  for (ParmVarDecl *Param : FD->parameters()) {
+    if (Param->hasDefaultArg()) {
+      return S.Diag(Param->getLocation(),
+                    diag::err_free_function_with_default_arg)
+             << Param->getSourceRange();
+    }
+  }
+  return false;
+}
+
 void SemaSYCL::ProcessFreeFunction(FunctionDecl *FD) {
   if (isFreeFunction(FD)) {
-    if (FD->isVariadic()) {
-      Diag(FD->getLocation(), diag::err_free_function_variadic_args);
+    if (CheckFreeFunctionDiagnostics(SemaRef, FD))
       return;
-    }
     SyclKernelDecompMarker DecompMarker(*this);
     SyclKernelFieldChecker FieldChecker(*this);
     SyclKernelUnionChecker UnionChecker(*this);

clang/test/SemaSYCL/free_function_negative.cpp

@@ -9,3 +9,33 @@ foo(int start, ...) { // expected-error {{free function kernel cannot be a varia
 [[__sycl_detail__::add_ir_attributes_function("sycl-single-task-kernel", 2)]] void 
 foo1(int start, ...) { // expected-error {{free function kernel cannot be a variadic function}}
 }
+
+// expected-error@+2 {{a function with a default argument value cannot be used to define SYCL free function kernel}}
+[[__sycl_detail__::add_ir_attributes_function("sycl-single-task-kernel", 2)]] void 
+singleTaskKernelDefaultValues(int Value = 1) {
+}
+
+// expected-error@+2 {{a function with a default argument value cannot be used to define SYCL free function kernel}}
+[[__sycl_detail__::add_ir_attributes_function("sycl-nd-range-kernel", 2)]] void 
+ndRangeKernelDefaultValues(int Value = 1) {
+}
+
+// expected-error@+2 {{a function with a default argument value cannot be used to define SYCL free function kernel}}
+[[__sycl_detail__::add_ir_attributes_function("sycl-single-task-kernel", 2)]] void  
+singleTaskKernelDefaultValues(int Ivalue = 1, unsigned int Uvalue = 3) {
+}
+
+// expected-error@+2 {{a function with a default argument value cannot be used to define SYCL free function kernel}}
+[[__sycl_detail__::add_ir_attributes_function("sycl-nd-range-kernel", 2)]] void 
+ndRangeKernelDefaultValues(int Ivalue = 1, unsigned int Uvalue = 3) {
+}
+
+// expected-error@+2 {{SYCL free function kernel should have return type 'void'}}
+[[__sycl_detail__::add_ir_attributes_function("sycl-single-task-kernel", 2)]] int 
+singleTaskKernelReturnType(int Value) {
+}
+
+// expected-error@+2 {{SYCL free function kernel should have return type 'void'}}
+[[__sycl_detail__::add_ir_attributes_function("sycl-nd-range-kernel", 2)]] int 
+ndRangeKernelReturnType(int Value) {
+}

sycl/test/extensions/free_function_kernels/free_function_kernels_diagnostics.cpp

@@ -18,11 +18,3 @@ SYCL_EXT_ONEAPI_FUNCTION_PROPERTY(syclexp::single_task_kernel)
 SYCL_EXT_ONEAPI_FUNCTION_PROPERTY(syclexp::nd_range_kernel<3>)
 void ndRangeKernelVariadic( // expected-error {{free function kernel cannot be a variadic function}}
     ...) {}
-
-// TODO: Add expected error when it will be implemented.
-SYCL_EXT_ONEAPI_FUNCTION_PROPERTY(syclexp::single_task_kernel)
-void singleTaskKernelDefaultValues(int Value = 1) {}
-
-// TODO: Add expected error when it will be implemented.
-SYCL_EXT_ONEAPI_FUNCTION_PROPERTY(syclexp::nd_range_kernel<1>)
-void ndRangeKernelDefaultValues(int Value = 1) {}