[CSGen] Diagnose subscripting a nil literal in CSGen.

Author: hborla

lib/Sema/CSDiag.cpp

@@ -1418,12 +1418,6 @@ bool FailureDiagnosis::diagnoseSubscriptErrors(SubscriptExpr *SE,
   if (!baseExpr) return true;
   auto baseType = CS.getType(baseExpr);
 
-  if (isa<NilLiteralExpr>(baseExpr)) {
-    diagnose(baseExpr->getLoc(), diag::cannot_subscript_nil_literal)
-      .highlight(baseExpr->getSourceRange());
-    return true;
-  }
-
   std::function<bool(ArrayRef<OverloadChoice>)> callback =
       [&](ArrayRef<OverloadChoice> candidates) -> bool {
     CalleeCandidateInfo calleeInfo(Type(), candidates, SE->hasTrailingClosure(),

lib/Sema/CSGen.cpp

@@ -1785,13 +1785,20 @@ namespace {
     }
 
     Type visitSubscriptExpr(SubscriptExpr *expr) {
+      auto &ctx = CS.getASTContext();
       ValueDecl *decl = nullptr;
       if (expr->hasDecl()) {
         decl = expr->getDecl().getDecl();
         if (decl->isInvalid())
           return Type();
       }
 
+      if (auto nilLiteral = dyn_cast<NilLiteralExpr>(expr->getBase())) {
+        ctx.Diags.diagnose(nilLiteral->getLoc(),
+                           diag::cannot_subscript_nil_literal);
+        return nullptr;
+      }
+
       return addSubscriptConstraints(expr, CS.getType(expr->getBase()),
                                      expr->getIndex(),
                                      decl, expr->getArgumentLabels(),