Sema: Report the use of reserved @cdecl names

Author: xymus

lib/Sema/TypeCheckAttr.cpp

@@ -2384,9 +2384,10 @@ void AttributeChecker::visitCDeclAttr(CDeclAttr *attr) {
              attr);
 
   // The standard library can use @_cdecl to implement runtime functions.
-  if (!canDeclareSymbolName(attr->Name, D->getModuleContext())) {
+  auto VD = dyn_cast<ValueDecl>(D);
+  if (VD && !canDeclareSymbolName(VD->getCDeclName(), D->getModuleContext())) {
       diagnose(attr->getLocation(), diag::reserved_runtime_symbol_name,
-               attr->Name);
+               VD->getCDeclName());
   }
 
   // @_cdecl was never accepted on enums. Keep the previous diagnostic.

test/attr/attr_cdecl_official.swift

@@ -198,3 +198,6 @@ func swiftErrorProtocol(e: Error) {}
 func anyParam(e:Any) {}
 // expected-error @-1 {{global function cannot be marked '@cdecl' because the type of the parameter cannot be represented in C}}
 // expected-note @-2 {{protocols cannot be represented in C}}
+
+@cdecl func swift_allocBox() {} // expected-warning {{symbol name 'swift_allocBox' is reserved for the Swift runtime and cannot be directly referenced without causing unpredictable behavior; this will become an error}}
+@cdecl(swift_allocObject) func swift_allocObject_renamed() {} // expected-warning {{symbol name 'swift_allocObject' is reserved for the Swift runtime and cannot be directly referenced without causing unpredictable behavior; this will become an error}}