[Associated type inference] Break recursive inference and fail predictably.

There was a path through associated type inference where we would end
up recording a type witness that contained an error, but for which we
had not reported that error, which would lead to downstream
crashes. Make sure that we reject such inferences.

And because it triggers once we fix this issue... make sure break
recursion when trying to resolve type witnesses lazily.

Fixes the crash in SR-6609 / rdar://problem/36038033, but we're still
failing to infer in those cases.

Author: DougGregor

lib/AST/ProtocolConformance.cpp

@@ -550,8 +550,9 @@ bool NormalProtocolConformance::hasTypeWitness(AssociatedTypeDecl *assocType,
   if (Loader)
     resolveLazyInfo();
 
-  if (TypeWitnesses.find(assocType) != TypeWitnesses.end()) {
-    return true;
+  auto found = TypeWitnesses.find(assocType);
+  if (found != TypeWitnesses.end()) {
+    return !found->getSecond().first.isNull();
   }
   if (resolver) {
     PrettyStackTraceRequirement trace("resolving", this, assocType);
@@ -594,6 +595,9 @@ NormalProtocolConformance::getTypeWitnessAndDecl(AssociatedTypeDecl *assocType,
   // Otherwise, resolve the type witness.
   PrettyStackTraceRequirement trace("resolving", this, assocType);
   assert(resolver && "Unable to resolve type witness");
+
+  // Block recursive resolution of this type witness.
+  TypeWitnesses[assocType] = { Type(), nullptr };
   resolver->resolveTypeWitness(this, assocType);
 
   known = TypeWitnesses.find(assocType);
@@ -606,7 +610,9 @@ void NormalProtocolConformance::setTypeWitness(AssociatedTypeDecl *assocType,
                                                TypeDecl *typeDecl) const {
   assert(getProtocol() == cast<ProtocolDecl>(assocType->getDeclContext()) &&
          "associated type in wrong protocol");
-  assert(TypeWitnesses.count(assocType) == 0 && "Type witness already known");
+  assert((TypeWitnesses.count(assocType) == 0 ||
+          TypeWitnesses[assocType].first.isNull()) &&
+         "Type witness already known");
   assert((!isComplete() || isInvalid()) && "Conformance already complete?");
   assert(!type->hasArchetype() && "type witnesses must be interface types");
   TypeWitnesses[assocType] = std::make_pair(type, typeDecl);

lib/Sema/TypeCheckProtocol.cpp

@@ -2655,8 +2655,12 @@ CheckTypeWitnessResult swift::checkTypeWitness(TypeChecker &tc, DeclContext *dc,
   auto *depTy = DependentMemberType::get(proto->getSelfInterfaceType(),
                                          assocType);
 
+  if (type->hasError())
+    return ErrorType::get(tc.Context);
+
   Type contextType = type->hasTypeParameter() ? dc->mapTypeIntoContext(type)
                                               : type;
+
   if (auto superclass = genericSig->getSuperclassBound(depTy)) {
     if (!superclass->isExactSuperclassOf(contextType))
       return superclass;
@@ -2780,7 +2784,8 @@ ResolveWitnessResult ConformanceChecker::resolveTypeWitnessViaLookup(
     [nonViable](NormalProtocolConformance *conformance) {
       auto &diags = conformance->getDeclContext()->getASTContext().Diags;
       for (auto candidate : nonViable) {
-        if (candidate.first->getDeclaredInterfaceType()->hasError())
+        if (candidate.first->getDeclaredInterfaceType()->hasError() ||
+            candidate.second.isError())
           continue;
 
         diags.diagnose(

lib/Sema/TypeCheckProtocol.h

@@ -78,7 +78,9 @@ class CheckTypeWitnessResult {
   bool isConformanceRequirement() const {
     return Requirement->isExistentialType();
   }
-
+  bool isError() const {
+    return Requirement->is<ErrorType>();
+  }
   explicit operator bool() const { return !Requirement.isNull(); }
 };
 

lib/Sema/TypeCheckProtocolInference.cpp

@@ -818,7 +818,7 @@ Type AssociatedTypeInference::computeDefaultTypeWitness(
 
   if (auto failed = checkTypeWitness(tc, dc, proto, assocType, defaultType)) {
     // Record the failure, if we haven't seen one already.
-    if (!failedDefaultedAssocType) {
+    if (!failedDefaultedAssocType && !failed.isError()) {
       failedDefaultedAssocType = defaultedAssocType;
       failedDefaultedWitness = defaultType;
       failedDefaultedResult = failed;
@@ -1680,6 +1680,9 @@ bool AssociatedTypeInference::diagnoseNoSolutions(
         diags.diagnose(assocType, diag::bad_associated_type_deduction,
                        assocType->getFullName(), proto->getFullName());
         for (const auto &failed : failedSet) {
+          if (failed.Result.isError())
+            continue;
+
           diags.diagnose(failed.Witness,
                          diag::associated_type_deduction_witness_failed,
                          failed.Requirement->getFullName(),

test/Constraints/same_types.swift

@@ -1,7 +1,7 @@
 // RUN: %target-typecheck-verify-swift -verify-ignore-unknown
 
 protocol Fooable {
-  associatedtype Foo
+  associatedtype Foo // expected-note{{protocol requires nested type 'Foo'; do you want to add it?}}
 
   var foo: Foo { get }
 }
@@ -148,7 +148,7 @@ rdar19137463(1)
 
 struct Brunch<U : Fooable> where U.Foo == X {}
 
-struct BadFooable : Fooable {
+struct BadFooable : Fooable { // expected-error{{type 'BadFooable' does not conform to protocol 'Fooable'}}
   typealias Foo = DoesNotExist // expected-error{{use of undeclared type 'DoesNotExist'}}
   var foo: Foo { while true {} }
 }

test/decl/nested/type_in_function.swift

@@ -23,7 +23,7 @@ func hasAClosure() {
 }
 
 protocol Racoon {
-  associatedtype Stripes
+  associatedtype Stripes // expected-note{{protocol requires nested type 'Stripes'; do you want to add it?}}
 }
 
 // Types inside generic functions -- not supported yet
@@ -117,7 +117,7 @@ struct OuterGenericStruct<A> {
   }
 
   func middleFunction() {
-    struct ConformingType : Racoon {
+    struct ConformingType : Racoon { // expected-error{{type 'ConformingType' does not conform to protocol 'Racoon'}}
     // expected-error@-1 {{type 'ConformingType' cannot be nested in generic function 'middleFunction()'}}
       typealias Stripes = A
     }

validation-test/compiler_crashers/28769-conformance-gettypewitness-assoctype-nullptr-isequal-type-conflicting-type-witne.swift renamed to validation-test/compiler_crashers_fixed/28769-conformance-gettypewitness-assoctype-nullptr-isequal-type-conflicting-type-witne.swift

@@ -5,6 +5,6 @@
 // See https://swift.org/LICENSE.txt for license information
 // See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
 
-// REQUIRES: asserts
-// RUN: not --crash %target-swift-frontend %s -emit-ir
+
+// RUN: not %target-swift-frontend %s -emit-ir
 protocol P{{}func a{}typealias e}struct A:P{typealias e:Self.a{}typealias e:Self.a{}typealias e:P

validation-test/compiler_crashers/28854-known-typewitnesses-end-didnt-resolve-witness.swift renamed to validation-test/compiler_crashers_fixed/28854-known-typewitnesses-end-didnt-resolve-witness.swift

@@ -5,8 +5,8 @@
 // See https://swift.org/LICENSE.txt for license information
 // See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
 
-// REQUIRES: asserts
-// RUN: not --crash %target-swift-frontend %s -emit-ir
+
+// RUN: not %target-swift-frontend %s -emit-ir
 @objc protocol A
 {
 typealias a