Index: lib/Parse/ParseDecl.cpp
===================================================================
--- lib/Parse/ParseDecl.cpp
+++ lib/Parse/ParseDecl.cpp
@@ -365,7 +365,8 @@
   // These may refer to the function arguments, but need to be parsed early to
   // participate in determining whether it's a redeclaration.
   std::unique_ptr<ParseScope> PrototypeScope;
-  if (AttrName->isStr("enable_if") && D && D->isFunctionDeclarator()) {
+  if (normalizeAttrName(AttrName->getName()) == "enable_if" &&
+      D && D->isFunctionDeclarator()) {
     DeclaratorChunk::FunctionTypeInfo FTI = D->getFunctionTypeInfo();
     PrototypeScope.reset(new ParseScope(this, Scope::FunctionPrototypeScope |
                                         Scope::FunctionDeclarationScope |
Index: test/Sema/enable_if.c
===================================================================
--- test/Sema/enable_if.c
+++ test/Sema/enable_if.c
@@ -77,6 +77,21 @@
 #endif
 }
 
+// Verify that the alternate spelling __enable_if__ works as well.
+int isdigit2(int c) __attribute__((overloadable));  // expected-note{{candidate function}}
+int isdigit2(int c) __attribute__((overloadable))  // expected-note{{candidate function has been explicitly made unavailable}}
+  __attribute__((__enable_if__(c <= -1 || c > 255, "'c' must have the value of an unsigned char or EOF")))
+  __attribute__((unavailable("'c' must have the value of an unsigned char or EOF")));
+
+void test4(int c) {
+  isdigit2(c);
+  isdigit2(10);
+#ifndef CODEGEN
+  isdigit2(-10);  // expected-error{{call to unavailable function 'isdigit2': 'c' must have the value of an unsigned char or EOF}}
+#endif
+}
+
+
 #ifndef CODEGEN
 __attribute__((enable_if(n == 0, "chosen when 'n' is zero"))) void f1(int n); // expected-error{{use of undeclared identifier 'n'}}