[ConstraintLocator] Add a new locator path element - ConformanceRequirement

Records protocol requirement associated with a particular type in
the constraint system, also useful to track parent conformance
for conditional requirements.

Author: xedin

include/swift/Sema/ConstraintLocator.h

@@ -762,6 +762,20 @@ class LocatorPathElt::ArgumentAttribute final : public StoredIntegerElement<1> {
   }
 };
 
+class LocatorPathElt::ConformanceRequirement final
+    : public StoredPointerElement<ProtocolDecl> {
+public:
+  ConformanceRequirement(ProtocolDecl *protocol)
+      : StoredPointerElement(PathElementKind::ConformanceRequirement,
+                             protocol) {}
+
+  ProtocolDecl *getRequirement() const { return getStoredPointer(); }
+
+  static bool classof(const LocatorPathElt *elt) {
+    return elt->getKind() == ConstraintLocator::ConformanceRequirement;
+  }
+};
+
 namespace details {
   template <typename CustomPathElement>
   class PathElement {

include/swift/Sema/ConstraintLocatorPathElts.def

@@ -195,6 +195,9 @@ CUSTOM_LOCATOR_PATH_ELT(ArgumentAttribute)
 /// The result of a chain of member accesses off an UnresolvedMemberExpr
 SIMPLE_LOCATOR_PATH_ELT(UnresolvedMemberChainResult)
 
+/// The conformance requirement associated with a type.
+CUSTOM_LOCATOR_PATH_ELT(ConformanceRequirement)
+
 #undef LOCATOR_PATH_ELT
 #undef CUSTOM_LOCATOR_PATH_ELT
 #undef SIMPLE_LOCATOR_PATH_ELT

lib/Sema/ConstraintLocator.cpp

@@ -66,6 +66,7 @@ unsigned LocatorPathElt::getNewSummaryFlags() const {
   case ConstraintLocator::KeyPathComponent:
   case ConstraintLocator::ConditionalRequirement:
   case ConstraintLocator::TypeParameterRequirement:
+  case ConstraintLocator::ConformanceRequirement:
   case ConstraintLocator::ImplicitlyUnwrappedDisjunctionChoice:
   case ConstraintLocator::DynamicLookupResult:
   case ConstraintLocator::ContextualType:
@@ -397,6 +398,14 @@ void ConstraintLocator::dump(SourceManager *sm, raw_ostream &out) const {
       break;
     }
 
+    case ConformanceRequirement: {
+      auto reqElt = elt.castTo<LocatorPathElt::ConformanceRequirement>();
+      out << "conformance requirement (";
+      reqElt.getRequirement()->dumpRef(out);
+      out << ")";
+      break;
+    }
+
     case ImplicitlyUnwrappedDisjunctionChoice:
       out << "implicitly unwrapped disjunction choice";
       break;