From 11cce3bcdc885391318ca4c4a64f205e4441562c Mon Sep 17 00:00:00 2001
From: Robert Widmann <devteam.codafi@gmail.com>
Date: Thu, 3 Dec 2015 22:12:43 -0500
Subject: [PATCH 1/2] Diagnose circular type aliases in protocol declarations

---
 include/swift/AST/Decl.h               | 11 +++++++++++
 lib/Sema/TypeCheckDecl.cpp             | 17 ++++++++++++++++-
 test/decl/protocol/req/recursion.swift |  4 ++++
 3 files changed, 31 insertions(+), 1 deletion(-)

diff --git a/include/swift/AST/Decl.h b/include/swift/AST/Decl.h
index be729431cf27e..029c44cc1a427 100644
--- a/include/swift/AST/Decl.h
+++ b/include/swift/AST/Decl.h
@@ -573,6 +573,9 @@ class alignas(1 << DeclAlignInBits) Decl {
     unsigned : NumTypeDeclBits;
 
     unsigned Recursive : 1;
+
+    /// Whether or not this declaration is currently being type-checked.
+    unsigned BeingTypeChecked : 1;
   };
   enum { NumAssociatedTypeDeclBits = NumTypeDeclBits + 1 };
   static_assert(NumAssociatedTypeDeclBits <= 32, "fits in an unsigned");
@@ -2605,6 +2608,14 @@ class AssociatedTypeDecl : public AbstractTypeParamDecl {
   void setIsRecursive() { AssociatedTypeDeclBits.Recursive = true; }
   bool isRecursive() { return AssociatedTypeDeclBits.Recursive; }
 
+  /// Whether the declaration is currently being validated.
+  bool isBeingTypeChecked() { return AssociatedTypeDeclBits.BeingTypeChecked; }
+
+  /// Toggle whether or not the declaration is being validated.
+  void setIsBeingTypeChecked(bool ibt = true) {
+    AssociatedTypeDeclBits.BeingTypeChecked = ibt;
+  }
+
   static bool classof(const Decl *D) {
     return D->getKind() == DeclKind::AssociatedType;
   }
diff --git a/lib/Sema/TypeCheckDecl.cpp b/lib/Sema/TypeCheckDecl.cpp
index 547e92fa264ce..cf9b537a7e19f 100644
--- a/lib/Sema/TypeCheckDecl.cpp
+++ b/lib/Sema/TypeCheckDecl.cpp
@@ -3173,7 +3173,7 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
         TAD->getUnderlyingTypeLoc().setInvalidType(TC.Context);
       } else if (TAD->getDeclContext()->isGenericContext()) {
         TAD->setInterfaceType(
-          TC.getInterfaceTypeFromInternalType(TAD->getDeclContext(),
+        TC.getInterfaceTypeFromInternalType(TAD->getDeclContext(),
                                               TAD->getType()));
       }
 
@@ -3192,6 +3192,19 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
   }
   
   void visitAssociatedTypeDecl(AssociatedTypeDecl *assocType) {
+    if (assocType->isBeingTypeChecked()) {
+
+      if (!assocType->hasType()) {
+        assocType->setInvalid();
+        assocType->overwriteType(ErrorType::get(TC.Context));
+      }
+
+      TC.diagnose(assocType->getLoc(), diag::circular_type_alias, assocType->getName());
+      return;
+    }
+
+    assocType->setIsBeingTypeChecked();
+
     TC.checkDeclAttributesEarly(assocType);
     if (!assocType->hasAccessibility())
       assocType->setAccessibility(assocType->getProtocol()->getFormalAccess());
@@ -3205,6 +3218,8 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
       defaultDefinition.setInvalidType(TC.Context);
     }
     TC.checkDeclAttributes(assocType);
+
+    assocType->setIsBeingTypeChecked(false);
   }
 
   bool checkUnsupportedNestedGeneric(NominalTypeDecl *NTD) {
diff --git a/test/decl/protocol/req/recursion.swift b/test/decl/protocol/req/recursion.swift
index 0a18a54b79d77..bc8090112d435 100644
--- a/test/decl/protocol/req/recursion.swift
+++ b/test/decl/protocol/req/recursion.swift
@@ -16,3 +16,7 @@ public struct S<A: P where A.T == S<A>> {}
 class X<T where T == X> { // expected-error{{same-type requirement makes generic parameter 'T' non-generic}}
     var type: T { return self.dynamicType } // expected-error{{use of undeclared type 'T'}}
 }
+
+protocol Y {
+  typealias Z = Z // expected-error{{type alias 'Z' circularly references itself}}
+}

From 9b68f5f6e34aa94099c4a460e5c08932ca36d089 Mon Sep 17 00:00:00 2001
From: Robert Widmann <devteam.codafi@gmail.com>
Date: Fri, 4 Dec 2015 02:24:55 -0500
Subject: [PATCH 2/2] Bump decl bits

---
 include/swift/AST/Decl.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/swift/AST/Decl.h b/include/swift/AST/Decl.h
index 029c44cc1a427..e6ee2fed757cd 100644
--- a/include/swift/AST/Decl.h
+++ b/include/swift/AST/Decl.h
@@ -577,7 +577,7 @@ class alignas(1 << DeclAlignInBits) Decl {
     /// Whether or not this declaration is currently being type-checked.
     unsigned BeingTypeChecked : 1;
   };
-  enum { NumAssociatedTypeDeclBits = NumTypeDeclBits + 1 };
+  enum { NumAssociatedTypeDeclBits = NumTypeDeclBits + 2 };
   static_assert(NumAssociatedTypeDeclBits <= 32, "fits in an unsigned");
 
   class ImportDeclBitfields {