Sema: Correct handling of Self-derived types in default witness matching

We want to use the context archetypes of the requirement in this case.

NFC until the next patch lands.

Author: slavapestov

lib/Sema/TypeCheckProtocol.cpp

@@ -934,11 +934,18 @@ matchWitness(TypeChecker &tc,
     // the required type and the witness type.
     cs.emplace(tc, dc, ConstraintSystemOptions());
 
-    Type model;
-    if (conformance)
-      model = conformance->getType();
-    else
-      model = dc->getDeclaredTypeInContext();
+    Type selfIfaceTy = proto->getProtocolSelf()->getDeclaredType();
+    Type selfTy;
+
+    // For a concrete conformance, use the conforming type as the
+    // base type of the requirement and witness lookup.
+    if (conformance) {
+      selfTy = conformance->getType();
+
+    // For a default witness, use the requirement's context archetype.
+    } else {
+      selfTy = ArchetypeBuilder::mapTypeIntoContext(dc, selfIfaceTy);
+    }
 
     // Open up the witness type.
     witnessType = witness->getInterfaceType();
@@ -950,7 +957,7 @@ matchWitness(TypeChecker &tc,
                        LocatorPathElt(ConstraintLocator::Witness, witness));
     if (witness->getDeclContext()->isTypeContext()) {
       std::tie(openedFullWitnessType, openWitnessType) 
-        = cs->getTypeOfMemberReference(model, witness,
+        = cs->getTypeOfMemberReference(selfTy, witness,
                                       /*isTypeReference=*/false,
                                       /*isDynamicResult=*/false,
                                       witnessLocator,
@@ -972,7 +979,7 @@ matchWitness(TypeChecker &tc,
     DeclContext *reqDC = req->getInnermostDeclContext();
     llvm::DenseMap<CanType, TypeVariableType *> replacements;
     std::tie(openedFullReqType, reqType)
-      = cs->getTypeOfMemberReference(model, req,
+      = cs->getTypeOfMemberReference(selfTy, req,
                                      /*isTypeReference=*/false,
                                      /*isDynamicResult=*/false,
                                      locator,
@@ -981,31 +988,45 @@ matchWitness(TypeChecker &tc,
 
     // Bind the associated types.
     for (const auto &replacement : replacements) {
-      if (auto gpType = replacement.first->getAs<GenericTypeParamType>()) {
-        // Record the type variable for 'Self'.
-        if (gpType->getDepth() == 0 && gpType->getIndex() == 0) {
-          cs->SelfTypeVar = replacement.second;
-          continue;
-        }
-        
-        // Replace any other type variable with the archetype within
-        // the requirement's context.
-        cs->addConstraint(ConstraintKind::Bind,
-                          replacement.second,
-                          ArchetypeBuilder::mapTypeIntoContext(reqDC, gpType),
-                          locator);
-
-      // Associated type of 'Self'.
-      } else if (auto assocType = getReferencedAssocTypeOfProtocol(
+
+      // If we have a concrete conformance, handle Self-derived type parameters
+      // differently.
+      if (conformance) {
+        if (replacement.first->is<GenericTypeParamType>()) {
+          // For a concrete conformance, we record the type variable for 'Self'
+          // without doing anything else. See the comment for SelfTypeVar in
+          // ConstraintSystem.h.
+          if (selfIfaceTy->isEqual(replacement.first)) {
+            cs->SelfTypeVar = replacement.second;
+            continue;
+          }
+
+          // Inner generic parameter of the requirement -- fall through below.
+
+        } else if (auto assocType = getReferencedAssocTypeOfProtocol(
                                                     replacement.first, proto)) {
-        if (conformance) {
+
+          // Look up the associated type witness.
           cs->addConstraint(ConstraintKind::Bind,
                             replacement.second,
                             conformance->getTypeWitness(assocType, nullptr)
                                  .getReplacement(),
                             locator);
+          continue;
+        } else {
+          continue;
         }
       }
+
+      // Replace any other type variable with the archetype within the
+      // requirement's context.
+      auto contextTy = ArchetypeBuilder::mapTypeIntoContext(
+          reqDC, replacement.first);
+
+      cs->addConstraint(ConstraintKind::Bind,
+                        replacement.second,
+                        contextTy,
+                        locator);
     }
     reqType = reqType->getRValueType();
 
@@ -1042,7 +1063,7 @@ matchWitness(TypeChecker &tc,
     if (openedFullWitnessType->hasTypeVariable()) {
       // Figure out the context we're substituting into.
       auto witnessDC = witness->getInnermostDeclContext();
-      
+
       // Compute the set of substitutions we'll need for the witness.
       solution->computeSubstitutions(witness->getInterfaceType(),
                                      witnessDC, openedFullWitnessType,
@@ -2311,7 +2332,8 @@ ConformanceChecker::resolveWitnessViaLookup(ValueDecl *requirement) {
 
   // If the requirement is optional, it's okay. We'll satisfy this via
   // our handling of default definitions.
-  // FIXME: also check for a default definition here.
+  //
+  // FIXME: revisit this once we get default definitions in protocol bodies.
   //
   // Treat 'unavailable' implicitly as if it were 'optional'.
   // The compiler will reject actual uses.
@@ -2400,7 +2422,7 @@ ResolveWitnessResult ConformanceChecker::resolveWitnessViaDerivation(
   return ResolveWitnessResult::ExplicitFailed;
 }
 
-// FIXME: use default definitions??
+// FIXME: revisit this once we get default implementations in protocol bodies.
 ResolveWitnessResult ConformanceChecker::resolveWitnessViaDefault(
                        ValueDecl *requirement) {
   assert(!isa<AssociatedTypeDecl>(requirement) && "Use resolveTypeWitnessVia*");
@@ -2413,8 +2435,6 @@ ResolveWitnessResult ConformanceChecker::resolveWitnessViaDefault(
     return ResolveWitnessResult::Success;
   }
 
-  // FIXME: Default definition.
-
   diagnoseOrDefer(requirement, true,
     [requirement](TypeChecker &tc, NormalProtocolConformance *conformance) {
     auto dc = conformance->getDeclContext();