[CSSimplify] Allow referencing typealias declarations via leading-dot syntax in generic context

Previously only static methods found on protocols were allowed but
it is reasonable to reference a typealias to perform an implicit call
to `.init` on its underlying type.

Resolves: rdar://88513939

Author: xedin

lib/Sema/CSSimplify.cpp

@@ -9727,7 +9727,7 @@ ConstraintSystem::simplifyUnresolvedMemberChainBaseConstraint(
       return SolutionKind::Solved;
 
     auto *memberRef = findResolvedMemberRef(memberLoc);
-    if (memberRef && memberRef->isStatic()) {
+    if (memberRef && (memberRef->isStatic() || isa<TypeAliasDecl>(memberRef))) {
       return simplifyConformsToConstraint(
           resultTy, baseTy, ConstraintKind::ConformsTo,
           getConstraintLocator(memberLoc, ConstraintLocator::MemberRefBase),

test/Constraints/static_members_on_protocol_in_generic_context.swift

@@ -325,3 +325,25 @@ func acceptStyle<S: Style>(_: S) {}
 
 acceptStyle(.formattedString(format: "hi")) // Ok
 acceptStyle(.number(42)) // Ok
+
+protocol Container {
+  associatedtype Content
+}
+
+struct Box<T>: Container {
+  typealias Content = T
+  init(_: Content) {}
+}
+
+extension Container {
+  // leading-dot syntax is going to use a typealias
+  typealias box = Box
+}
+
+// rdar://88513939 - Allow to call init through a typealias using leading-dot syntax in generic context
+func test_leading_dot_syntax_with_typelias() {
+  func test<T: Container>(_: T) {}
+
+  test(Container.box(1)) // Ok
+  test(.box(1)) // Ok `Container.box(1)` means `Box.init(1)`
+}