[Type checker] Fix a crash triggered from index-while-building.

Author: DougGregor

lib/Sema/CSSimplify.cpp

@@ -4783,7 +4783,8 @@ performMemberLookup(ConstraintKind constraintKind, DeclName memberName,
     // as representing "dynamic lookup" unless it's a direct call
     // to such subscript (in that case label is expected to match).
     if (auto *subscript = dyn_cast<SubscriptDecl>(cand)) {
-      if (::hasDynamicMemberLookupAttribute(instanceTy,
+      if (memberLocator &&
+          ::hasDynamicMemberLookupAttribute(instanceTy,
                                             DynamicMemberLookupCache) &&
           isValidKeyPathDynamicMemberLookup(subscript, TC)) {
         auto info = getArgumentInfo(memberLocator);

test/Index/Store/dynamic-member-lookup-crash.swift

@@ -0,0 +1,24 @@
+// Ensure that we don't crash looking for default implementations during
+// indexing.
+
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend -index-store-path %t/idx -o %t/file.o -typecheck -primary-file %s -verify
+
+@dynamicMemberLookup
+protocol B {
+  associatedtype Value
+
+  var value: Value { get }
+  
+  subscript<Subject>(dynamicMember dynamicMember: WritableKeyPath<Value, Subject>) -> Value { get }
+}
+
+extension B {
+    subscript<Subject>(dynamicMember dynamicMember: WritableKeyPath<Value, Subject>) -> Value {
+    get { value }
+  }
+}
+
+struct Foo<Value>: B {
+  var value: Value  
+}