[CSFix] Account for nested structural errors related to function argument mismatches

When it comes to contextual mismatches `ContextualType` is not guaranteed
to be the last element in the patch since contextual type could be a function
too which could would have `contextual type -> function argument` path.

Author: xedin

lib/Sema/CSFix.cpp

@@ -262,11 +262,15 @@ ContextualMismatch *ContextualMismatch::create(ConstraintSystem &cs, Type lhs,
 /// and the contextual type.
 static Optional<std::tuple<ContextualTypePurpose, Type, Type>>
 getStructuralTypeContext(ConstraintSystem &cs, ConstraintLocator *locator) {
-  if (auto contextualType = cs.getContextualType(locator->getAnchor())) {
-    if (auto *anchor = simplifyLocatorToAnchor(locator))
-      return std::make_tuple(cs.getContextualTypePurpose(locator->getAnchor()),
-                             cs.getType(anchor),
-                             contextualType);
+  if (locator->findLast<LocatorPathElt::ContextualType>()) {
+    assert(locator->isLastElement<LocatorPathElt::ContextualType>() ||
+           locator->isLastElement<LocatorPathElt::FunctionArgument>());
+
+    auto *anchor = locator->getAnchor();
+    auto contextualType = cs.getContextualType(anchor);
+    auto exprType = cs.getType(anchor);
+    return std::make_tuple(cs.getContextualTypePurpose(anchor), exprType,
+                           contextualType);
   } else if (auto argApplyInfo = cs.getFunctionArgApplyInfo(locator)) {
     return std::make_tuple(CTP_CallArgument,
                            argApplyInfo->getArgType(),

test/Constraints/sr10595.swift

@@ -15,6 +15,6 @@ class B<Y> : Nested { // expected-error {{type 'B<Y>' does not conform to protoc
 
 class C<M> {
     func cFunc(_ a: A<M>) {
-        let function: (B<Int>, ReferenceWritableKeyPath<M, Int>) -> Void = a.f // expected-error {{cannot convert value of type '(_, WritableKeyPath<M, _.U>) -> ()' to specified type '(B<Int>, ReferenceWritableKeyPath<M, Int>) -> Void'}}
+        let function: (B<Int>, ReferenceWritableKeyPath<M, Int>) -> Void = a.f // expected-error {{cannot convert value of type '(B<Int>, WritableKeyPath<M, B<Int>.U>) -> ()' to specified type '(B<Int>, ReferenceWritableKeyPath<M, Int>) -> Void'}}
     }
 }