Sema: Fix crash with associated type <=> typealias circularity

Author: slavapestov

lib/Sema/TypeCheckNameLookup.cpp

@@ -459,6 +459,13 @@ LookupTypeResult TypeChecker::lookupMemberType(DeclContext *dc,
       // Use the type witness.
       auto concrete = conformance->getConcrete();
       Type memberType = concrete->getTypeWitness(assocType, this);
+
+      // This is the only case where NormalProtocolConformance::
+      // getTypeWitnessAndDecl() returns a null type.
+      if (concrete->getState() ==
+          ProtocolConformanceState::CheckingTypeWitnesses)
+        continue;
+
       assert(memberType && "Missing type witness?");
 
       // If we haven't seen this type result yet, add it to the result set.

validation-test/compiler_crashers/28751-membertype-missing-type-witness.swift renamed to validation-test/compiler_crashers_fixed/28751-membertype-missing-type-witness.swift

@@ -6,5 +6,5 @@
 // See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
 
 // REQUIRES: asserts
-// RUN: not --crash %target-swift-frontend %s -emit-ir
+// RUN: not %target-swift-frontend %s -emit-ir
 {class a:P{{}typealias a:Self.a}protocol P{typealias a