Don't open existentials for library builtin shims

Author: DougGregor

lib/Sema/CSSimplify.cpp

@@ -1421,6 +1421,18 @@ namespace {
     OptionSet<OpenedExistentialAdjustmentFlags>;
 }
 
+/// Determine if this function is part of the _isUnique family of functions in
+/// the standard library.
+static bool isStdlibUniqueFunction(ValueDecl *callee) {
+  if (!callee->isStdlibDecl())
+    return false;
+
+  auto baseName = callee->getName().getBaseName().userFacingName();
+  return baseName ==  "_isUnique" || baseName == "_isUnique_native" ||
+         baseName == "_COWBufferForReading" ||
+         baseName == "_unsafeDowncastToAnyObject";
+}
+
 /// Determine whether we should open up the existential argument to the
 /// given parameters.
 ///
@@ -1457,6 +1469,14 @@ shouldOpenExistentialCallArgument(
     return None;
 
   case DeclTypeCheckingSemantics::Normal:
+    // _isUnique and friends are special because opening an existential when
+    // calling them would make them non-unique.
+    // FIXME: Borrowing properly from the existential box would probably
+    // eliminate this.
+    if (isStdlibUniqueFunction(callee))
+      return None;
+    break;
+
   case DeclTypeCheckingSemantics::WithoutActuallyEscaping:
     break;
   }