Use existing diagnostic and address other minor comments

Author: schittir

clang/include/clang/Basic/Attr.td

@@ -1644,7 +1644,7 @@ def DeviceKernel : DeclOrTypeAttr {
 def SYCLExternal : InheritableAttr {
   let Spellings = [Clang<"sycl_external">];
   let Subjects = SubjectList<[Function], ErrorDiag>;
-  let LangOpts = [SYCLDevice];
+  let LangOpts = [SYCLHost, SYCLDevice];
   let Documentation = [SYCLExternalDocs];
 }
 

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -12884,10 +12884,9 @@ def err_sycl_attribute_invalid_linkage : Error<
   "'sycl_external' can only be applied to functions with external linkage">;
 def err_sycl_attribute_avoid_main : Error<
   "'sycl_external' cannot be applied to the 'main' function">;
-def err_sycl_attribute_avoid_deleted_function : Error<
-  "'sycl_external' cannot be applied to an explicitly deleted function">;
-def err_sycl_attribute_missing_on_first_decl
-    : Error<"'sycl_external' must be applied to the first declaration">;
+def err_sycl_attribute_avoid_deleted_function
+    : Error<"'sycl_external' cannot be applied to an explicitly deleted "
+            "function">;
 
 // SYCL kernel entry point diagnostics
 def err_sycl_entry_point_invalid : Error<

clang/lib/Sema/SemaDecl.cpp

@@ -4084,16 +4084,18 @@ bool Sema::MergeFunctionDecl(FunctionDecl *New, NamedDecl *&OldD, Scope *S,
            diag::note_carries_dependency_missing_first_decl) << 0/*Function*/;
     }
 
-    // SYCL spec 2020
-    //   The first declaration of a function with external linkage must
-    //   specify sycl_external attribute.
-    //   Subsequent declarations may optionally specify this attribute.
+    // SYCL 2020 section 5.10.1, "SYCL functions and member functions linkage":
+    //   When a function is declared with sycl_external, that attribute must be
+    //   used on the first declaration of that function in the translation unit.
+    //   Redeclarations of the function in the same translation unit may
+    //   optionally use sycl_external, but this is not required.
     if (LangOpts.SYCLIsDevice) {
       const SYCLExternalAttr *SEA = New->getAttr<SYCLExternalAttr>();
       if (SEA && !Old->hasAttr<SYCLExternalAttr>()) {
-        Diag(SEA->getLocation(), diag::err_sycl_attribute_missing_on_first_decl)
+        Diag(SEA->getLocation(), diag::err_attribute_missing_on_first_decl)
             << SEA;
         Diag(Old->getLocation(), diag::note_previous_declaration);
+        New->dropAttr<SYCLExternalAttr>();
       }
     }
 

clang/test/CodeGenSYCL/kernel-caller-entry-point.cpp

@@ -100,8 +100,7 @@ int main() {
 
 // Verify that SYCL kernel caller functions are emitted for each device target.
 //
-// main() shouldn't be emitted in device code. It is not annotated with
-// sycl_kernel_entry_point or sycl_external attributes.
+// main() shouldn't be emitted in device code.
 // CHECK-NOT: define {{[a-z_ ]*}}noundef i32 @main() #0
 
 // IR for the SYCL kernel caller function generated for

clang/test/SemaSYCL/sycl-external-attr.cpp

@@ -18,7 +18,7 @@ namespace {
 // The first declaration of a SYCL external function is required to have this attribute.
 int foo(); // expected-note {{previous declaration is here}}
 
-[[clang::sycl_external]] int foo(); // expected-error {{'sycl_external' must be applied to the first declaration}}
+[[clang::sycl_external]] int foo(); // expected-error {{'clang::sycl_external' attribute does not appear on the first declaration}}
 
 // Subsequent declrations of a SYCL external function may optionally specify this attribute.
 [[clang::sycl_external]] int boo();