Simplest fix to a dependency bug.

Better commenting-out

lib/AST/AbstractSourceFileDepGraphFactory.cpp

@@ -79,14 +79,30 @@ void AbstractSourceFileDepGraphFactory::addAUsedDecl(
     const DependencyKey &defKey, const DependencyKey &useKey) {
   auto *defNode =
       g.findExistingNodeOrCreateIfNew(defKey, None, false /* = !isProvides */);
+
   // If the depended-upon node is defined in this file, then don't
   // create an arc to the user, when the user is the whole file.
   // Otherwise, if the defNode's type-body fingerprint changes,
   // the whole file will be marked as dirty, losing the benefit of the
   // fingerprint.
-  if (defNode->getIsProvides() &&
-      useKey.getKind() == NodeKind::sourceFileProvide)
-    return;
+
+  //  if (defNode->getIsProvides() &&
+  //      useKey.getKind() == NodeKind::sourceFileProvide)
+  //    return;
+
+  // Turns out the above three lines cause miscompiles, so comment them out
+  // for now. We might want them back if we can change the inputs to this
+  // function to be more precise.
+
+  // Example of a miscompile:
+  // In main.swift
+  // func foo(_: Any) { print("Hello Any") }
+  //    foo(123)
+  // Then add the following line to another file:
+  // func foo(_: Int) { print("Hello Int") }
+  // Although main.swift needs to get recompiled, the commented-out code below
+  // prevents that.
+
   auto nullableUse = g.findExistingNode(useKey);
   assert(nullableUse.isNonNull() && "Use must be an already-added provides");
   auto *useNode = nullableUse.get();