[Diagnostics] Add a tailored note for passing nil to incompatible argument position

New note mentions both expected argument type and its position
and anchors to the affected overload choice.

Author: xedin

include/swift/AST/DiagnosticsSema.def

@@ -425,6 +425,9 @@ ERROR(cannot_convert_argument_value_anyobject,none,
       (Type, Type))
 ERROR(cannot_convert_argument_value_nil,none,
       "'nil' is not compatible with expected argument type %0", (Type))
+NOTE(note_incompatible_argument_value_nil_at_pos,none,
+     "'nil' is not compatible with expected argument type %0 at position #%1",
+     (Type, unsigned))
 
 ERROR(cannot_convert_condition_value,none,
       "cannot convert value of type %0 to expected condition type %1",

lib/Sema/CSDiagnostics.cpp

@@ -2375,11 +2375,26 @@ bool ContextualFailure::diagnoseAsError() {
 }
 
 bool ContextualFailure::diagnoseAsNote() {
-  auto overload = getCalleeOverloadChoiceIfAvailable(getLocator());
+  auto *locator = getLocator();
+
+  auto overload = getCalleeOverloadChoiceIfAvailable(locator);
   if (!(overload && overload->choice.isDecl()))
     return false;
 
   auto *decl = overload->choice.getDecl();
+
+  if (auto *anchor = getAsExpr(getAnchor())) {
+    anchor = anchor->getSemanticsProvidingExpr();
+
+    if (isa<NilLiteralExpr>(anchor)) {
+      auto argLoc =
+          locator->castLastElementTo<LocatorPathElt::ApplyArgToParam>();
+      emitDiagnosticAt(decl, diag::note_incompatible_argument_value_nil_at_pos,
+                       getToType(), argLoc.getArgIdx() + 1);
+      return true;
+    }
+  }
+
   emitDiagnosticAt(decl, diag::found_candidate_type, getFromType());
   return true;
 }