[ConstraintSystem] Add a special locator for keypath dynamic member lookup

New locator is going to store pointer to the base keypath declaration.
This information is useful to determine whether such keypath is appropriate
to access underlying properties/subscripts e.g. `WritableKeyPath` cannot be
used to access read-only storage.

Author: xedin

lib/Sema/ConstraintLocator.cpp

@@ -45,6 +45,10 @@ void ConstraintLocator::Profile(llvm::FoldingSetNodeID &id, Expr *anchor,
       id.AddPointer(elt.getWitness());
       break;
 
+    case KeyPathDynamicMember:
+      id.AddPointer(elt.getKeyPath());
+      break;
+
     case ApplyArgument:
     case ApplyFunction:
     case FunctionArgument:
@@ -266,6 +270,10 @@ void ConstraintLocator::dump(SourceManager *sm, raw_ostream &out) {
     case SynthesizedArgument:
       out << " synthesized argument #" << llvm::utostr(elt.getValue());
       break;
+
+    case KeyPathDynamicMember:
+      out << " keypath dynamic member lookup";
+      break;
     }
   }
 

lib/Sema/ConstraintLocator.h

@@ -127,6 +127,8 @@ class ConstraintLocator : public llvm::FoldingSetNode {
     ContextualType,
     /// The missing argument synthesized by the solver.
     SynthesizedArgument,
+    /// The member looked up via keypath based dynamic lookup.
+    KeyPathDynamicMember,
   };
 
   /// Determine the number of numeric values used for the given path
@@ -165,6 +167,7 @@ class ConstraintLocator : public llvm::FoldingSetNode {
     case TupleElement:
     case KeyPathComponent:
     case SynthesizedArgument:
+    case KeyPathDynamicMember:
       return 1;
 
     case TypeParameterRequirement:
@@ -221,6 +224,7 @@ class ConstraintLocator : public llvm::FoldingSetNode {
     case DynamicLookupResult:
     case ContextualType:
     case SynthesizedArgument:
+    case KeyPathDynamicMember:
       return 0;
 
     case FunctionArgument:
@@ -241,6 +245,7 @@ class ConstraintLocator : public llvm::FoldingSetNode {
       StoredRequirement,
       StoredWitness,
       StoredGenericSignature,
+      StoredKeyPathDynamicMemberBase,
       StoredKindAndValue
     };
 
@@ -290,6 +295,10 @@ class ConstraintLocator : public llvm::FoldingSetNode {
         : storage((reinterpret_cast<uintptr_t>(sig) >> 3)),
           storedKind(StoredGenericSignature) {}
 
+    PathElement(const NominalTypeDecl *keyPath)
+        : storage((reinterpret_cast<uintptr_t>(keyPath) >> 3)),
+          storedKind(StoredKeyPathDynamicMemberBase) {}
+
     friend class ConstraintLocator;
 
   public:
@@ -369,6 +378,10 @@ class ConstraintLocator : public llvm::FoldingSetNode {
       return PathElement(SynthesizedArgument, position);
     }
 
+    static PathElement getKeyPathDynamicMember(const NominalTypeDecl *base) {
+      return PathElement(base);
+    }
+
     /// Retrieve the kind of path element.
     PathElementKind getKind() const {
       switch (static_cast<StoredKind>(storedKind)) {
@@ -384,6 +397,9 @@ class ConstraintLocator : public llvm::FoldingSetNode {
       case StoredGenericSignature:
         return OpenedGeneric;
 
+      case StoredKeyPathDynamicMemberBase:
+        return KeyPathDynamicMember;
+
       case StoredKindAndValue:
         return decodeStorage(storage).first;
       }
@@ -443,6 +459,13 @@ class ConstraintLocator : public llvm::FoldingSetNode {
       return reinterpret_cast<GenericSignature *>(storage << 3);
     }
 
+    NominalTypeDecl *getKeyPath() const {
+      assert((static_cast<StoredKind>(storedKind) ==
+              StoredKeyPathDynamicMemberBase) &&
+             "Is not a keypath dynamic member");
+      return reinterpret_cast<NominalTypeDecl *>(storage << 3);
+    }
+
     /// Return the summary flags for this particular element.
     unsigned getNewSummaryFlags() const {
       return getSummaryFlagsForPathElement(getKind());
@@ -459,6 +482,10 @@ class ConstraintLocator : public llvm::FoldingSetNode {
     bool isSynthesizedArgument() const {
       return getKind() == PathElementKind::SynthesizedArgument;
     }
+
+    bool isKeyPathDynamicMember() const {
+      return getKind() == PathElementKind::KeyPathDynamicMember;
+    }
   };
 
   /// Return the summary flags for an entire path.