AST: Fix infinite recursion with recursive same-type constraints

We diagnose and flag these in finalize(), but we can encounter one
even earlier through resolveDependentMemberTypes(). Do the same
thing there that we already do in EquivalenceClass::getTypeInContext().

Fixes <rdar://problem/45328122>, <https://bugs.swift.org/browse/SR-9022>.

Author: slavapestov

lib/AST/GenericSignatureBuilder.cpp

@@ -3875,6 +3875,12 @@ static Type resolveDependentMemberTypes(GenericSignatureBuilder &builder,
       if (equivClass->recursiveConcreteType)
         return ErrorType::get(Type(type));
 
+      // Prevent recursive substitution.
+      equivClass->recursiveConcreteType = true;
+      SWIFT_DEFER {
+        equivClass->recursiveConcreteType = false;
+      };
+
       return resolveDependentMemberTypes(builder, equivClass->concreteType);
     }
 

validation-test/compiler_crashers_2_fixed/0180-sr9022.swift

@@ -0,0 +1,6 @@
+// RUN: not %target-swift-frontend -typecheck %s
+
+class Graph<V>: Collection {
+  typealias Iterator = AnyIterator<V>
+  typealias Index = Int
+}