Sema: More conservative 'tautological binding' check

Instead of computing the reduced type of the witness upfront and
then checking for canonical equality in type matching, check for
reduced equality in type matching.

This restores the old behavior and prevents us from considering too
many protocol extension witnesses, while fixing the request cycle
that motivated the change to instead match against the reduced type
of the witness.

Fixes rdar://problem/122589094, rdar://problem/122596633.

Author: slavapestov

lib/Sema/AssociatedTypeInference.cpp

@@ -1382,46 +1382,65 @@ enum class InferenceCandidateKind {
 static InferenceCandidateKind checkInferenceCandidate(
     std::pair<AssociatedTypeDecl *, Type> *result,
     NormalProtocolConformance *conformance,
-    DeclContext *witnessDC,
+    ValueDecl *witness,
     Type selfTy) {
   auto &ctx = selfTy->getASTContext();
 
+  // The unbound form of `Self.A`.
+  auto selfAssocTy = DependentMemberType::get(selfTy, result->first->getName());
+  auto genericSig = witness->getInnermostDeclContext()
+      ->getGenericSignatureOfContext();
+
+  if (ctx.LangOpts.EnableExperimentalAssociatedTypeInference) {
+    // If the witness is in a protocol extension for a completely unrelated
+    // protocol that doesn't declare an associated type with the same name as
+    // the one we are trying to infer, then it will never be tautological.
+    if (!genericSig->isValidTypeParameter(selfAssocTy))
+      return InferenceCandidateKind::Good;
+  }
+
+  // A tautological binding is one where the left-hand side has the same
+  // reduced type as the right-hand side in the generic signature of the
+  // witness.
   auto isTautological = [&](Type t) -> bool {
+    if (ctx.LangOpts.EnableExperimentalAssociatedTypeInference) {
+
     auto dmt = t->getAs<DependentMemberType>();
     if (!dmt)
       return false;
-    if (!associatedTypesAreSameEquivalenceClass(dmt->getAssocType(),
-                                                result->first))
-      return false;
 
-    Type typeInContext;
-    if (ctx.LangOpts.EnableExperimentalAssociatedTypeInference) {
-
-    typeInContext = selfTy;
+    return genericSig->areReducedTypeParametersEqual(dmt, selfAssocTy);
 
     } else {
 
-    typeInContext =
-      conformance->getDeclContext()->mapTypeIntoContext(conformance->getType());
-
-    }
+    auto dmt = t->getAs<DependentMemberType>();
+    if (!dmt)
+      return false;
+    if (!associatedTypesAreSameEquivalenceClass(dmt->getAssocType(),
+                                                result->first))
+      return false;
 
+    Type typeInContext =
+      conformance->getDeclContext()->mapTypeIntoContext(conformance->getType());
     if (!dmt->getBase()->isEqual(typeInContext))
       return false;
 
     return true;
+
+    }
   };
 
   // Self.X == Self.X doesn't give us any new information, nor does it
   // immediately fail.
   if (isTautological(result->second)) {
-    auto *dmt = result->second->castTo<DependentMemberType>();
-
-    auto selfAssocTy = DependentMemberType::get(selfTy, dmt->getAssocType());
+    // FIXME: This should be getInnermostDeclContext()->getGenericSignature(),
+    // but that might introduce new ambiguities in existing code so we need
+    // to be careful.
+    auto genericSig = witness->getDeclContext()->getGenericSignatureOfContext();
 
     // If we have a same-type requirement `Self.X == Self.Y`,
     // introduce a binding `Self.X := Self.Y`.
-    for (auto &reqt : witnessDC->getGenericSignatureOfContext().getRequirements()) {
+    for (auto &reqt : genericSig.getRequirements()) {
       switch (reqt.getKind()) {
       case RequirementKind::SameShape:
         llvm_unreachable("Same-shape requirement not supported here");
@@ -1432,6 +1451,45 @@ static InferenceCandidateKind checkInferenceCandidate(
         break;
 
       case RequirementKind::SameType:
+        if (ctx.LangOpts.EnableExperimentalAssociatedTypeInference) {
+
+        auto matches = [&](Type t) {
+          if (auto *dmt = t->getAs<DependentMemberType>()) {
+            return (dmt->getName() == result->first->getName() &&
+                    dmt->getBase()->isEqual(selfTy));
+          }
+
+          return false;
+        };
+
+        // If we have a tautological binding, check if the witness generic
+        // signature has a same-type requirement `Self.A == Self.X` or
+        // `Self.X == Self.A`, where `A` is an associated type with the same
+        // name as the one we're trying to infer, and `X` is some other type
+        // parameter.
+        Type other;
+        if (matches(reqt.getFirstType())) {
+          other = reqt.getSecondType();
+        } else if (matches(reqt.getSecondType())) {
+          other = reqt.getFirstType();
+        } else {
+          break;
+        }
+
+        if (other->isTypeParameter() &&
+            other->getRootGenericParam()->isEqual(selfTy)) {
+          result->second = other;
+          LLVM_DEBUG(llvm::dbgs() << "++ we can same-type to:\n";
+                     result->second->dump(llvm::dbgs()));
+          return InferenceCandidateKind::Good;
+
+        }
+
+        } else {
+
+        auto *dmt = result->second->castTo<DependentMemberType>();
+        auto selfAssocTy = DependentMemberType::get(selfTy, dmt->getAssocType());
+
         Type other;
         if (reqt.getFirstType()->isEqual(selfAssocTy)) {
           other = reqt.getSecondType();
@@ -1443,18 +1501,9 @@ static InferenceCandidateKind checkInferenceCandidate(
 
         if (auto otherAssoc = other->getAs<DependentMemberType>()) {
           if (otherAssoc->getBase()->isEqual(selfTy)) {
-            DependentMemberType *otherDMT;
-            if (ctx.LangOpts.EnableExperimentalAssociatedTypeInference) {
-
-            otherDMT = otherAssoc;
-  
-            } else {
-
-            otherDMT = DependentMemberType::get(dmt->getBase(),
+            auto *otherDMT = DependentMemberType::get(dmt->getBase(),
                                                 otherAssoc->getAssocType());
 
-            }
-
             result->second = result->second.transform([&](Type t) -> Type{
               if (t->isEqual(dmt))
                 return otherDMT;
@@ -1465,6 +1514,8 @@ static InferenceCandidateKind checkInferenceCandidate(
             return InferenceCandidateKind::Good;
           }
         }
+
+        }
         break;
       }
     }
@@ -1609,8 +1660,7 @@ AssociatedTypeInference::getPotentialTypeWitnessesFromRequirement(
       // itself involve unresolved type witnesses.
       if (selfTy) {
         // Handle Self.X := Self.X and Self.X := G<Self.X>.
-        switch (checkInferenceCandidate(&result, conformance,
-                                        witness->getDeclContext(), selfTy)) {
+        switch (checkInferenceCandidate(&result, conformance, witness, selfTy)) {
         case InferenceCandidateKind::Good:
           // The "good" case is something like `Self.X := Self.Y`.
           break;
@@ -1864,37 +1914,27 @@ static Type getWitnessTypeForMatching(NormalProtocolConformance *conformance,
   auto proto = conformance->getProtocol();
   auto selfTy = proto->getSelfInterfaceType();
 
-  // Get the reduced type of the witness. This rules our certain tautological
-  // inferences below.
-  if (ctx.LangOpts.EnableExperimentalAssociatedTypeInference) {
-    if (auto genericSig = witness->getInnermostDeclContext()
-            ->getGenericSignatureOfContext()) {
-      type = genericSig.getReducedType(type);
-      type = genericSig->getSugaredType(type);
-    }
-  }
-
-  // Remap associated types that reference other protocols into this
-  // protocol.
-  type = type.transformRec([proto](TypeBase *type) -> llvm::Optional<Type> {
-    if (auto depMemTy = dyn_cast<DependentMemberType>(type)) {
-      if (depMemTy->getAssocType() &&
-          depMemTy->getAssocType()->getProtocol() != proto) {
-        if (auto *assocType = proto->getAssociatedType(depMemTy->getName())) {
-          auto origProto = depMemTy->getAssocType()->getProtocol();
-          if (proto->inheritsFrom(origProto))
-            return Type(DependentMemberType::get(depMemTy->getBase(),
-                                                 assocType));
+  if (!ctx.LangOpts.EnableExperimentalAssociatedTypeInference) {
+    // Remap associated types that reference other protocols into this
+    // protocol.
+    auto resultType = Type(type).transformRec([proto](TypeBase *type)
+                                                 -> llvm::Optional<Type> {
+      if (auto depMemTy = dyn_cast<DependentMemberType>(type)) {
+        if (depMemTy->getAssocType() &&
+            depMemTy->getAssocType()->getProtocol() != proto) {
+          if (auto *assocType = proto->getAssociatedType(depMemTy->getName())) {
+            auto origProto = depMemTy->getAssocType()->getProtocol();
+            if (proto->inheritsFrom(origProto))
+              return Type(DependentMemberType::get(depMemTy->getBase(),
+                                                   assocType));
+          }
         }
       }
-    }
 
-    return llvm::None;
-  });
-
-  if (!ctx.LangOpts.EnableExperimentalAssociatedTypeInference) {
-    auto resultType = type.subst(QueryTypeSubstitutionMap{substitutions},
-                                 LookUpConformanceInModule(module));
+      return llvm::None;
+    });
+    resultType = resultType.subst(QueryTypeSubstitutionMap{substitutions},
+                                  LookUpConformanceInModule(module));
     if (!resultType->hasError()) return resultType;
 
     // Map error types with original types *back* to the original, dependent type.
@@ -1916,9 +1956,28 @@ static Type getWitnessTypeForMatching(NormalProtocolConformance *conformance,
     if (!rootParam->isEqual(selfTy))
       return type;
 
+    // Remap associated types that reference other protocols into this
+    // protocol.
+    auto substType = Type(type).transformRec([proto](TypeBase *type)
+                                                 -> llvm::Optional<Type> {
+      if (auto depMemTy = dyn_cast<DependentMemberType>(type)) {
+        if (depMemTy->getAssocType() &&
+            depMemTy->getAssocType()->getProtocol() != proto) {
+          if (auto *assocType = proto->getAssociatedType(depMemTy->getName())) {
+            auto origProto = depMemTy->getAssocType()->getProtocol();
+            if (proto->inheritsFrom(origProto))
+              return Type(DependentMemberType::get(depMemTy->getBase(),
+                                                   assocType));
+          }
+        }
+      }
+
+      return llvm::None;
+    });
+
     // Replace Self with the concrete conforming type.
-    auto substType = Type(type).subst(QueryTypeSubstitutionMap{substitutions},
-                                      LookUpConformanceInModule(module));
+    substType = substType.subst(QueryTypeSubstitutionMap{substitutions},
+                                LookUpConformanceInModule(module));
 
     // If we don't have enough type witnesses, leave it abstract.
     if (substType->hasError())

test/decl/protocol/req/rdar122589094.swift

@@ -0,0 +1,35 @@
+// RUN: %target-typecheck-verify-swift -enable-experimental-associated-type-inference
+// RUN: %target-typecheck-verify-swift -disable-experimental-associated-type-inference
+
+protocol P1 {
+  associatedtype A
+
+  func f1(_: C) -> A
+  func f2(_: A, _: C)
+
+  typealias C = S1<Self>
+}
+
+struct S1<T> {}
+
+protocol P2: P1 where A == B {
+  associatedtype B
+
+  func g1(_: C) -> B
+  func g2(_: B, _: C)
+}
+
+extension P2 {
+  func f1(_: C) -> B { fatalError() }
+  func f2(_: B, _: C) { fatalError() }
+}
+
+extension P2 {
+  func g2(_: B, _: C) {}
+}
+
+struct S2: P2 {
+  func g1(_: C) -> Int {
+    fatalError()
+  }
+}

test/decl/protocol/req/rdar122596633-1.swift

@@ -0,0 +1,49 @@
+// RUN: %target-typecheck-verify-swift -disable-experimental-associated-type-inference
+// RUN: %target-typecheck-verify-swift -enable-experimental-associated-type-inference
+
+public protocol P1 {
+    associatedtype A = Void
+
+    func makeA() -> A
+    func consumeA(a: inout A)
+}
+
+extension P1 where A == Void {
+    // Don't consider this witness in the 'S2: P1' conformance below.
+    public func makeA() -> A { fatalError() }
+}
+
+public struct S1: P1 {}
+
+public protocol P2: P1 where A == B.A {
+    associatedtype B: P1
+    var base: B { get }
+}
+
+extension P2 {
+    public func makeA() -> B.A { fatalError() }
+    public func consumeA(a: inout B.A) {}
+}
+
+extension S1: P2 {
+    public var base: S2 { fatalError() }
+}
+
+public struct S2 {}
+
+public struct S3 {}
+
+extension S2: P1 {
+    public typealias A = S3
+}
+
+public protocol P3: P1 where A == S3 {}
+
+extension P3 {
+    public func makeA() -> A { fatalError() }
+    public func consumeA(a: inout A) {}
+}
+
+extension S2: P3 {}
+
+let x: S3.Type = S2.A.self

test/decl/protocol/req/rdar122596633-2.swift

@@ -0,0 +1,32 @@
+// RUN: %target-typecheck-verify-swift -disable-experimental-associated-type-inference
+// RUN: %target-typecheck-verify-swift -enable-experimental-associated-type-inference
+
+public protocol P<A> {
+    associatedtype A
+    associatedtype B: P
+
+    func makeA() -> A
+    var b: B { get }
+}
+
+extension P where A == B.A {
+    public func makeA() -> B.A {
+        fatalError()
+    }
+}
+
+public struct S: P {
+    public var b: some P<Int> {
+        return G<Int>()
+    }
+}
+
+public struct G<A>: P {
+    public func makeA() -> A { fatalError() }
+    public var b: Never { fatalError() }
+}
+
+extension Never: P {
+    public func makeA() -> Never { fatalError() }
+    public var b: Never { fatalError() }
+}