[CSDiagnostics] Generic argument mismatches should always mention the container types

(cherry picked from commit a8146d6)

Author: xedin

lib/Sema/CSDiagnostics.cpp

@@ -904,16 +904,44 @@ bool GenericArgumentsMismatchFailure::diagnoseAsError() {
   // before pointer types could be compared.
   auto locator = getLocator();
   auto path = locator->getPath();
-  unsigned toDrop = 0;
-  for (const auto &elt : llvm::reverse(path)) {
-    if (!elt.is<LocatorPathElt::OptionalPayload>())
-      break;
 
-    // Disregard optional payload element to look at its source.
-    ++toDrop;
+  // If there are generic types involved, we need to find
+  // the outermost generic types and report on them instead
+  // of their arguments.
+  // For example:
+  //
+  //    <expr> -> contextual type
+  //           -> generic type S<[Int]>
+  //           -> generic type S<[String]>
+  //           -> generic argument #0
+  //
+  // Is going to have from/to types as `[Int]` and `[String]` but
+  // the diagnostic should mention `S<[Int]>` and `S<[String]>`
+  // because it refers to a contextual type location.
+  if (locator->isLastElement<LocatorPathElt::GenericArgument>()) {
+    for (unsigned i = 0; i < path.size(); ++i) {
+      if (auto genericType = path[i].getAs<LocatorPathElt::GenericType>()) {
+        ASSERT(i + 1 < path.size());
+
+        fromType = resolveType(genericType->getType());
+        toType = resolveType(
+            path[i + 1].castTo<LocatorPathElt::GenericType>().getType());
+        break;
+      }
+    }
   }
 
-  path = path.drop_back(toDrop);
+  while (!path.empty()) {
+    auto last = path.back();
+    if (last.is<LocatorPathElt::OptionalPayload>() ||
+        last.is<LocatorPathElt::GenericType>() ||
+        last.is<LocatorPathElt::GenericArgument>()) {
+      path = path.drop_back();
+      continue;
+    }
+
+    break;
+  }
 
   std::optional<Diag<Type, Type>> diagnostic;
   if (path.empty()) {