[4.0] Collected fixes

include/swift/AST/GenericSignatureBuilder.h

@@ -869,7 +869,7 @@ class GenericSignatureBuilder::RequirementSource final
            "RequirementSource kind/storageKind mismatch");
 
     storage.type = type.getPointer();
-    if (kind == ProtocolRequirement)
+    if (isProtocolRequirement())
       getTrailingObjects<ProtocolDecl *>()[0] = protocol;
     if (hasTrailingWrittenRequirementLoc)
       getTrailingObjects<WrittenRequirementLoc>()[0] = writtenReqLoc;
@@ -979,6 +979,11 @@ class GenericSignatureBuilder::RequirementSource final
            llvm::function_ref<bool(PotentialArchetype *,
                                    const RequirementSource *)> visitor) const;
 
+  /// Whether this source is a requirement in a protocol.
+  bool isProtocolRequirement() const {
+    return kind == ProtocolRequirement || kind == InferredProtocolRequirement;
+  }
+
   /// Whether the requirement is inferred or derived from an inferred
   /// requirement.
   bool isInferredRequirement() const;

lib/AST/GenericSignature.cpp

@@ -823,8 +823,7 @@ ConformanceAccessPath GenericSignature::getConformanceAccessPath(
   buildPath = [&](GenericSignature *sig, const RequirementSource *source,
                   ProtocolDecl *conformingProto, Type rootType) {
     // Each protocol requirement is a step along the path.
-    if (source->kind == RequirementSource::ProtocolRequirement ||
-        source->kind == RequirementSource::InferredProtocolRequirement) {
+    if (source->isProtocolRequirement()) {
       // Follow the rest of the path to derive the conformance into which
       // this particular protocol requirement step would look.
       auto inProtocol = source->getProtocolDecl();

lib/AST/GenericSignatureBuilder.cpp

@@ -613,7 +613,7 @@ ProtocolDecl *RequirementSource::getProtocolDecl() const {
     return nullptr;
 
   case StorageKind::StoredType:
-    if (kind == ProtocolRequirement)
+    if (isProtocolRequirement())
       return getTrailingObjects<ProtocolDecl *>()[0];
     return nullptr;
 
@@ -637,7 +637,7 @@ SourceLoc RequirementSource::getLoc() const {
   // for a particular requirement, rather than turning on/off location info.
   // Locations that fall into this category should be advisory, emitted via
   // notes rather than as the normal location.
-  if (kind == ProtocolRequirement && parent &&
+  if (isProtocolRequirement() && parent &&
       parent->kind != RequirementSignatureSelf)
     return parent->getLoc();
 
@@ -668,8 +668,7 @@ SourceLoc RequirementSource::getLoc() const {
 static unsigned sourcePathLength(const RequirementSource *source) {
   unsigned count = 0;
   for (; source; source = source->parent) {
-    if (source->kind == RequirementSource::ProtocolRequirement ||
-        source->kind == RequirementSource::InferredProtocolRequirement)
+    if (source->isProtocolRequirement())
       ++count;
   }
   return count;
@@ -942,7 +941,7 @@ bool FloatingRequirementSource::isRecursive(
   llvm::SmallSet<std::pair<CanType, ProtocolDecl *>, 4> visitedAssocReqs;
   for (auto storedSource = storage.dyn_cast<const RequirementSource *>();
        storedSource; storedSource = storedSource->parent) {
-    if (storedSource->kind != RequirementSource::ProtocolRequirement)
+    if (!storedSource->isProtocolRequirement())
       continue;
 
     if (!visitedAssocReqs.insert(

lib/Sema/TypeChecker.cpp

@@ -578,10 +578,14 @@ static void typeCheckFunctionsAndExternalDecls(TypeChecker &TC) {
   // Now that all types have been finalized, run any delayed
   // circularity checks.
   // This has been written carefully to fail safe + finitely if
-  // for some reason a type gets re-delayed.
+  // for some reason a type gets re-delayed in a non-assertions
+  // build in an otherwise successful build.
+  // Types can be redelayed in a failing build because we won't
+  // type-check required declarations from different files.
   for (size_t i = 0, e = TC.DelayedCircularityChecks.size(); i != e; ++i) {
     TC.checkDeclCircularity(TC.DelayedCircularityChecks[i]);
-    assert(e == TC.DelayedCircularityChecks.size() &&
+    assert((e == TC.DelayedCircularityChecks.size() ||
+            TC.Context.hadError()) &&
            "circularity checking for type was re-delayed!");
   }
   TC.DelayedCircularityChecks.clear();

test/Sema/Inputs/circularity_multifile_error_helper.swift

@@ -0,0 +1,5 @@
+struct External {
+  var member: Something
+}
+
+struct OtherExternal {}

test/Sema/circularity_multifile_error.swift

@@ -0,0 +1,11 @@
+// RUN: %target-swift-frontend -emit-silgen -verify -primary-file %s %S/Inputs/circularity_multifile_error_helper.swift
+
+// SR-4594
+
+struct A {
+  var b: AnUndefinedType // expected-error {{use of undeclared type 'AnUndefinedType'}}
+}
+
+struct B {
+  var a : External
+}