Sema: Add ProtocolDecl::DefaultWitnesses

NFC for now, since the code that uses this is not checked in yet.

Author: slavapestov

include/swift/AST/Decl.h

@@ -27,6 +27,7 @@
 #include "swift/AST/TypeAlignments.h"
 #include "swift/Basic/OptionalEnum.h"
 #include "swift/Basic/Range.h"
+#include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/DenseSet.h"
 #include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/Support/TrailingObjects.h"
@@ -3350,6 +3351,8 @@ class ProtocolDecl : public NominalTypeDecl {
 
   ArrayRef<ProtocolDecl *> InheritedProtocols;
 
+  llvm::DenseMap<ValueDecl *, ConcreteDeclRef> DefaultWitnesses;
+
   /// True if the protocol has requirements that cannot be satisfied (e.g.
   /// because they could not be imported from Objective-C).
   unsigned HasMissingRequirements : 1;
@@ -3494,6 +3497,23 @@ class ProtocolDecl : public NominalTypeDecl {
     HasMissingRequirements = newValue;
   }
 
+  /// Returns the default witness for a requirement, or nullptr if there is
+  /// no default.
+  ConcreteDeclRef getDefaultWitness(ValueDecl *requirement) {
+    auto found = DefaultWitnesses.find(requirement);
+    if (found == DefaultWitnesses.end())
+      return nullptr;
+    return found->second;
+  }
+
+  /// Record the default witness for a requirement.
+  void setDefaultWitness(ValueDecl *requirement, ConcreteDeclRef witness) {
+    assert(witness);
+    auto pair = DefaultWitnesses.insert(std::make_pair(requirement, witness));
+    assert(pair.second && "Already have a default witness!");
+    (void) pair;
+  }
+
   /// Set the list of inherited protocols.
   void setInheritedProtocols(ArrayRef<ProtocolDecl *> protocols) {
     assert(!InheritedProtocolsSet && "protocols already set");

lib/Sema/TypeCheckProtocol.cpp

@@ -350,6 +350,12 @@ namespace {
     /// \brief Associated type substitutions needed to match the witness.
     SmallVector<Substitution, 2> WitnessSubstitutions;
 
+    ConcreteDeclRef getWitness(ASTContext &ctx) const {
+      if (WitnessSubstitutions.empty())
+        return Witness;
+      return ConcreteDeclRef(ctx, Witness, WitnessSubstitutions);
+    }
+
     /// Classify the provided optionality issues for use in diagnostics.
     /// FIXME: Enumify this
     unsigned classifyOptionalityIssues(ValueDecl *requirement) const {
@@ -1849,12 +1855,7 @@ void ConformanceChecker::recordWitness(ValueDecl *requirement,
   }
 
   // Record this witness in the conformance.
-  ConcreteDeclRef witness;
-  if (match.WitnessSubstitutions.empty())
-    witness = match.Witness;
-  else
-    witness = ConcreteDeclRef(TC.Context, match.Witness,
-                              match.WitnessSubstitutions);
+  ConcreteDeclRef witness = match.getWitness(TC.Context);
   Conformance->setWitness(requirement, witness);
 
   // Synthesize accessors for the protocol witness table to use.