[CSDiagnostics] Look into tuple expressions to find requirement failure location

If the requirement failure is related to an assignment destination
analyze the source of the assignment and if it's a tuple, find the
element and return its type.

Author: xedin

lib/Sema/CSDiagnostics.cpp

@@ -242,8 +242,17 @@ Type RequirementFailure::getOwnerType() const {
   // to convert source to destination, which means that
   // owner type is actually not an assignment expression
   // itself but its source.
-  if (auto *assignment = getAsExpr<AssignExpr>(anchor))
+  if (auto *assignment = getAsExpr<AssignExpr>(anchor)) {
     anchor = assignment->getSrc();
+    // If locator points to a tuple element, let's dig that up.
+    // Situations like `<<dest>> = (v, 2)` where `v` has a requirement failure.
+    if (auto tupleEltIdx =
+            getLocator()->findFirst<LocatorPathElt::AnyTupleElement>()) {
+      if (auto *tuple = getAsExpr<TupleExpr>(anchor)) {
+        return getType(tuple->getElement(tupleEltIdx->getIndex()));
+      }
+    }
+  }
 
   return getType(anchor)->getInOutObjectType()->getMetatypeInstanceType();
 }