Sema: Fix crash when override checking encounters circularity

getInterfaceType() will return an ErrorType if the declaration is currently
being validated, which can happen if override checking triggers associated
type inference.

I believe this is <rdar://problem/47220065>.

Author: slavapestov

lib/Sema/TypeCheckDeclOverride.cpp

@@ -77,15 +77,16 @@ Type swift::getMemberTypeForComparison(ASTContext &ctx, ValueDecl *member,
   SubscriptDecl *subscript = dyn_cast_or_null<SubscriptDecl>(abstractStorage);
 
   auto memberType = member->getInterfaceType();
+  if (memberType->is<ErrorType>())
+    return memberType;
+
   if (derivedDecl) {
     auto *dc = derivedDecl->getDeclContext();
     auto owningType = dc->getDeclaredInterfaceType();
     assert(owningType);
 
     memberType = owningType->adjustSuperclassMemberDeclType(member, derivedDecl,
                                                             memberType);
-    if (memberType->hasError())
-      return memberType;
   }
 
   if (method) {

test/decl/circularity.swift

@@ -55,3 +55,19 @@ extension SIMD3 {
 extension SIMD3 where SIMD3.Scalar == Float {
     static let pickMe = SIMD3(.pickMe)
 }
+
+// Test case with circular overrides
+protocol P {
+    associatedtype A
+    func run(a: A)
+}
+
+class C1 {
+    func run(a: Int) {}
+}
+
+class C2: C1, P {
+    override func run(a: A) {}
+    // expected-error@-1 {{circular reference}}
+    // expected-note@-2 2{{through reference here}}
+}