[ConstraintSystem] De-prioritize missing member fix when base is contextual

Impact is higher if the base is expected to be inferred from context,
because a failure to find a member ultimately means that base type is
not a match in this case.

Author: xedin

lib/Sema/CSSimplify.cpp

@@ -5397,7 +5397,12 @@ ConstraintSystem::SolutionKind ConstraintSystem::simplifyMemberConstraint(
 
       auto *fix =
           DefineMemberBasedOnUse::create(*this, baseTy, member, locator);
-      if (recordFix(fix))
+      // Impact is higher if the base is expected to be inferred from context,
+      // because a failure to find a member ultimately means that base type is
+      // not a match in this case.
+      auto impact =
+          locator->findLast<LocatorPathElt::UnresolvedMember>() ? 2 : 1;
+      if (recordFix(fix, impact))
         return SolutionKind::Error;
 
       // Allow member type to default to `Any` to make it possible to form

lib/Sema/ConstraintLocator.h

@@ -359,6 +359,7 @@ class ConstraintLocator : public llvm::FoldingSetNode {
     class GenericParameter;
     class OpenedGeneric;
     class KeyPathDynamicMember;
+    class UnresolvedMember;
 
     PathElement(PathElementKind kind)
       : storage(encodeStorage(kind, 0)), storedKind(StoredKindAndValue)
@@ -910,6 +911,15 @@ class LocatorPathElt::KeyPathDynamicMember final : public LocatorPathElt {
   }
 };
 
+class LocatorPathElt::UnresolvedMember final : public LocatorPathElt {
+public:
+  UnresolvedMember() : LocatorPathElt(PathElementKind::UnresolvedMember, 0) {}
+
+  static bool classof(const LocatorPathElt *elt) {
+    return elt->getKind() == ConstraintLocator::UnresolvedMember;
+  }
+};
+
 /// A simple stack-only builder object that constructs a
 /// constraint locator without allocating memory.
 ///