Diagnose circular type aliases in protocol declarations

Author: CodaFi

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;
   }

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) {

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}}
+}