[GSB] Reimplement self-derived checking for conformance constraints.

The general self-derived check doesn't really make sense for
conformance constraints, because we want to distinguish among
different protocol conformances.

Author: DougGregor

include/swift/AST/GenericSignatureBuilder.h

@@ -889,6 +889,11 @@ class GenericSignatureBuilder::RequirementSource final
   /// derived requirement ceases to hold.
   bool isSelfDerivedSource(PotentialArchetype *pa) const;
 
+  /// Determine whether a requirement \c pa: proto, when formed from this
+  /// requirement source, is dependent on itself.
+  bool isSelfDerivedConformance(PotentialArchetype *pa,
+                                ProtocolDecl *proto) const;
+
   /// Retrieve a source location that corresponds to the requirement.
   SourceLoc getLoc() const;
 

lib/AST/GenericSignatureBuilder.cpp

@@ -271,6 +271,144 @@ bool RequirementSource::isSelfDerivedSource(PotentialArchetype *pa) const {
   return false;
 }
 
+/// Replace 'Self' in the given dependent type (\c depTy) with the given
+/// potential archetype, producing a new potential archetype that refers to
+/// the nested type. This limited operation makes sure that it does not
+/// create any new potential archetypes along the way, so it should only be
+/// used in cases where we're reconstructing something that we know exists.
+static PotentialArchetype *replaceSelfWithPotentialArchetype(
+                             PotentialArchetype *selfPA, Type depTy) {
+  if (auto depMemTy = depTy->getAs<DependentMemberType>()) {
+    // Recurse to produce the potential archetype for the base.
+    auto basePA = replaceSelfWithPotentialArchetype(selfPA,
+                                                    depMemTy->getBase());
+
+    PotentialArchetype *nestedPAByName = nullptr;
+
+    auto assocType = depMemTy->getAssocType();
+    auto name = depMemTy->getName();
+    auto findNested = [&](PotentialArchetype *pa) -> PotentialArchetype * {
+      const auto &nested = pa->getNestedTypes();
+      auto found = nested.find(name);
+
+      if (found == nested.end()) return nullptr;
+      if (found->second.empty()) return nullptr;
+
+      // Note that we've found a nested PA by name.
+      if (!nestedPAByName) {
+        nestedPAByName = found->second.front();
+      }
+
+      // If we don't have an associated type to look for, we're done.
+      if (!assocType) return nestedPAByName;
+
+      // Look for a nested PA matching the associated type.
+      for (auto nestedPA : found->second) {
+        if (nestedPA->getResolvedAssociatedType() == assocType)
+          return nestedPA;
+      }
+
+      return nullptr;
+    };
+
+    // First, look in the base potential archetype for the member we want.
+    if (auto result = findNested(basePA))
+      return result;
+
+    // Otherwise, look elsewhere in the equivalence class of the base potential
+    // archetype.
+    for (auto otherBasePA : basePA->getEquivalenceClassMembers()) {
+      if (otherBasePA == basePA) continue;
+
+      if (auto result = findNested(otherBasePA))
+        return result;
+    }
+
+    assert(nestedPAByName && "Didn't find the associated type we wanted");
+    return nestedPAByName;
+  }
+
+  assert(depTy->is<GenericTypeParamType>() && "missing Self?");
+  return selfPA;
+}
+
+bool RequirementSource::isSelfDerivedConformance(PotentialArchetype *currentPA,
+                                                 ProtocolDecl *proto) const {
+  /// Keep track of all of the requirements we've seen along the way. If
+  /// we see the same requirement twice, it's a self-derived conformance.
+  llvm::DenseSet<std::pair<PotentialArchetype *, ProtocolDecl *>>
+    constraintsSeen;
+
+  // Note that we've now seen a new constraint, returning true if we've seen
+  // it before.
+  auto addConstraint = [&](PotentialArchetype *pa, ProtocolDecl *proto) {
+    return !constraintsSeen.insert({pa->getRepresentative(), proto}).second;
+  };
+
+  // Insert our end state.
+  constraintsSeen.insert({currentPA->getRepresentative(), proto});
+
+  // Follow from the root of the
+  std::function<PotentialArchetype *(const RequirementSource *source)>
+    followFromRoot;
+
+  bool sawProtocolRequirement = false;
+  followFromRoot = [&](const RequirementSource *source) -> PotentialArchetype *{
+    // Handle protocol requirements.
+    if (source->kind == ProtocolRequirement) {
+      sawProtocolRequirement = true;
+
+      // Compute the base potential archetype.
+      auto basePA = followFromRoot(source->parent);
+      if (!basePA) return nullptr;
+
+      // The base potential archetype must conform to the protocol in which
+      // this requirement results.
+      if (addConstraint(basePA, source->getProtocolDecl()))
+        return nullptr;
+
+      // If there's no stored type, return the base.
+      if (!source->getStoredType()) return basePA;
+
+      // Follow the dependent type in the protocol requirement.
+      return replaceSelfWithPotentialArchetype(basePA, source->getStoredType());
+    }
+
+    if (source->kind == Parent) {
+      // Compute the base potential archetype.
+      auto basePA = followFromRoot(source->parent);
+      if (!basePA) return nullptr;
+
+      // Add on this associated type.
+      return replaceSelfWithPotentialArchetype(
+               basePA,
+               source->getAssociatedType()->getDeclaredInterfaceType());
+    }
+
+    if (source->parent)
+      return followFromRoot(source->parent);
+
+    // We are at a root, so the root potential archetype is our result.
+    auto rootPA = source->getRootPotentialArchetype();
+
+    // If we haven't seen a protocol requirement, we're done.
+    if (!sawProtocolRequirement) return rootPA;
+
+    // The root archetype might be a nested type, which implies constraints
+    // for each of the protocols of the associated types referenced (if any).
+    for (auto pa = rootPA; pa->getParent(); pa = pa->getParent()) {
+      if (auto assocType = pa->getResolvedAssociatedType()) {
+        if (addConstraint(pa->getParent(), assocType->getProtocol()))
+          return nullptr;
+      }
+    }
+
+    return rootPA;
+  };
+
+  return followFromRoot(this) == nullptr;
+}
+
 #define REQUIREMENT_SOURCE_FACTORY_BODY(ProfileArgs, ConstructorArgs,      \
                                         NumProtocolDecls, WrittenReq)      \
   llvm::FoldingSetNodeID nodeID;                                           \
@@ -3312,7 +3450,18 @@ void GenericSignatureBuilder::checkConformanceConstraints(
     return;
 
   for (auto &entry : equivClass->conformsTo) {
-    checkConstraintList<ProtocolDecl *>(
+    // Remove self-derived constraints.
+    assert(!entry.second.empty() && "No constraints to work with?");
+    entry.second.erase(
+      std::remove_if(entry.second.begin(), entry.second.end(),
+                     [&](const Constraint<ProtocolDecl *> &constraint) {
+                       return constraint.source->isSelfDerivedConformance(
+                                constraint.archetype, entry.first);
+                     }),
+      entry.second.end());
+    assert(!entry.second.empty() && "All constraints were self-derived!");
+
+    checkConstraintList<ProtocolDecl *, ProtocolDecl *>(
       genericParams, entry.second,
       [](const Constraint<ProtocolDecl *> &constraint) {
         return true;
@@ -3323,7 +3472,9 @@ void GenericSignatureBuilder::checkConformanceConstraints(
       },
       None,
       diag::redundant_conformance_constraint,
-      diag::redundant_conformance_here);
+      diag::redundant_conformance_here,
+      [](ProtocolDecl *proto) { return proto; },
+      /*removeSelfDerived=*/false);
   }
 }
 

test/IRGen/same_type_constraints.swift

@@ -19,7 +19,7 @@ public extension P where Foo == DefaultFoo<Self> {
 
 // <rdar://26873036> IRGen crash with derived class declaring same-type constraint on constrained associatedtype.
 public class C1<T: Equatable> { }
-public class C2<T: Equatable, U: P where T == U.Foo>: C1<T> {}
+public class C2<T: Equatable, U: P>: C1<T> where T == U.Foo {}
 
 // CHECK: define{{( protected)?}} swiftcc void @_T021same_type_constraints2C1CfD
 

test/decl/class/circular_inheritance.swift

@@ -5,7 +5,7 @@ class B : A { } // expected-note{{class 'B' declared here}}
 class A : C { } // expected-note{{class 'A' declared here}}
 
 class TrivialCycle : TrivialCycle {} // expected-error{{circular class inheritance TrivialCycle}}
-protocol P : P {} // expected-error 3{{circular protocol inheritance P}}
+protocol P : P {} // expected-error 2{{circular protocol inheritance P}}
 
 class Isomorphism : Automorphism { }
 class Automorphism : Automorphism { } // expected-error{{circular class inheritance Automorphism}}

test/decl/protocol/protocols.swift

@@ -82,10 +82,8 @@ protocol CircleStart : CircleEnd { func circle_start() } // expected-error 2{{ci
 // expected-note@-1{{protocol 'CircleStart' declared here}}
 protocol CircleEnd : CircleMiddle { func circle_end()} // expected-note{{protocol 'CircleEnd' declared here}}
 
-// expected-warning@+2{{redundant conformance constraint 'Self': 'CircleTrivial'}}
-// expected-note@+1{{conformance constraint 'Self': 'CircleTrivial' implied here}}
 protocol CircleEntry : CircleTrivial { }
-protocol CircleTrivial : CircleTrivial { } // expected-error 3{{circular protocol inheritance CircleTrivial}}
+protocol CircleTrivial : CircleTrivial { } // expected-error 2{{circular protocol inheritance CircleTrivial}}
 
 struct Circle {
   func circle_start() {}