[ConstraintSystem] Move unviable keypath dynamic member check into performMemberLookup

Author: xedin

lib/Sema/CSDiag.cpp

@@ -811,6 +811,7 @@ void FailureDiagnosis::diagnoseUnviableLookupResults(
   if (sameProblem) {
     switch (firstProblem) {
     case MemberLookupResult::UR_LabelMismatch:
+    case MemberLookupResult::UR_WritableKeyPathOnReadOnlyMember:
       break;
     case MemberLookupResult::UR_UnavailableInExistential:
       diagnose(loc, diag::could_not_use_member_on_existential,

lib/Sema/CSSimplify.cpp

@@ -3826,6 +3826,28 @@ performMemberLookup(ConstraintKind constraintKind, DeclName memberName,
       }
     }
 
+    // Check whether this is overload choice found via keypath
+    // based dynamic member lookup. Since it's unknown upfront
+    // what kind of declaration lookup is going to find, let's
+    // double check here that given keypath is appropriate for it.
+    if (memberLocator) {
+      auto path = memberLocator->getPath();
+      if (!path.empty() && path.back().isKeyPathDynamicMember()) {
+        auto *keyPath = path.back().getKeyPath();
+        if (auto *storage = dyn_cast<AbstractStorageDecl>(decl)) {
+          // If this is an attempt to access read-only member via
+          // writable key path, let's fail this choice early.
+          if (isReadOnlyKeyPathComponent(storage) &&
+              keyPath == getASTContext().getWritableKeyPathDecl()) {
+            result.addUnviable(
+                candidate,
+                MemberLookupResult::UR_WritableKeyPathOnReadOnlyMember);
+            return;
+          }
+        }
+      }
+    }
+
     // Otherwise, we're good, add the candidate to the list.
     result.addViable(candidate);
   };
@@ -4193,6 +4215,11 @@ fixMemberRef(ConstraintSystem &cs, Type baseTy,
     case MemberLookupResult::UR_MutatingGetterOnRValue:
     case MemberLookupResult::UR_LabelMismatch:
     case MemberLookupResult::UR_UnavailableInExistential:
+    // TODO(diagnostics): Add a new fix that is suggests to
+    // add `subscript(dynamicMember: KeyPath<T, U>)`
+    // overload here, that would help if such subscript has
+    // not been provided.
+    case MemberLookupResult::UR_WritableKeyPathOnReadOnlyMember:
       break;
     }
   }

lib/Sema/ConstraintSystem.cpp

@@ -1988,29 +1988,6 @@ void ConstraintSystem::resolveOverload(ConstraintLocator *locator,
       }
     }
 
-    // Check whether this is overload choice found via keypath
-    // based dynamic member lookup. Since it's unknown upfront
-    // what kind of declaration lookup is going to find, let's
-    // double check here that given keypath is appropriate for it.
-    auto path = locator->getPath();
-    if (!path.empty() && path.back().isKeyPathDynamicMember()) {
-      auto *keyPath = path.back().getKeyPath();
-      if (auto *storage = dyn_cast<AbstractStorageDecl>(decl)) {
-        // If this is an attempt to access read-only member via
-        // writable key path, let's fail this choice early.
-        //
-        // TODO(diagnostics): Add a new fix that is suggests to
-        // add `subscript(dynamicMember: WritableKeyPath<T, U>)`
-        // overload here, instead of failing.
-        if (isReadOnlyKeyPathComponent(storage) &&
-            keyPath == getASTContext().getWritableKeyPathDecl()) {
-          failedConstraint = Constraint::create(*this, ConstraintKind::Bind,
-                                                boundType, refType, locator);
-          return;
-        }
-      }
-    }
-
     // Check whether applying this overload would result in invalid
     // partial function application e.g. partial application of
     // mutating method or initializer.

lib/Sema/ConstraintSystem.h

@@ -872,6 +872,12 @@ struct MemberLookupResult {
 
     /// The member is inaccessible (e.g. a private member in another file).
     UR_Inaccessible,
+
+    /// This is a `WritableKeyPath` being used to look up read-only member,
+    /// used in situations involving dynamic member lookup via keypath,
+    /// because it's not known upfront what access capability would the
+    /// member have.
+    UR_WritableKeyPathOnReadOnlyMember,
   };
 
   /// This is a list of considered (but rejected) candidates, along with a