[Sema] Add assertion to diagnoseImplicitSelfUseInClosure

The only cases where we don't have a closure type
here should now be for implicit expressions and
macro expansions. Eventually we ought to stop
doing `typeCheckExpression` in CSApply entirely,
which would eliminate this special case.

Author: hamishknight

lib/Sema/MiscDiagnostics.cpp

@@ -2524,11 +2524,25 @@ static void diagnoseImplicitSelfUseInClosure(const Expr *E,
   AbstractClosureExpr *ACE = nullptr;
   if (DC->isLocalContext()) {
     while (DC->getParent()->isLocalContext() && !ACE) {
-      // FIXME: This is happening too early, because closure->getType() isn't set.
-      if (auto *closure = dyn_cast<AbstractClosureExpr>(DC))
-        if (closure->getType())
+      if (auto *closure = dyn_cast<AbstractClosureExpr>(DC)) {
+        // FIXME: We have a couple of calls to typeCheckExpression remaining in
+        // CSApply that can result in running this logic before the solution has
+        // been applied to the parent expression. This should only happen for
+        // implicit code and macro expansions though.
+        auto isInGeneratedBuffer = [&]() -> bool {
+          auto loc = E->getStartLoc();
+          if (!loc)
+            return false;
+          auto buffer = ctx.SourceMgr.findBufferContainingLoc(loc);
+          return ctx.SourceMgr.hasGeneratedSourceInfo(buffer);
+        };
+        DEBUG_ASSERT(closure->getType() || E->isImplicit() ||
+                     isInGeneratedBuffer());
+        if (closure->getType()) {
           if (DiagnoseWalker::isClosureRequiringSelfQualification(closure))
             ACE = const_cast<AbstractClosureExpr *>(closure);
+        }
+      }
       DC = DC->getParent();
     }
   }