[Type checker] Allow 'Self' reference in a lazy initializer.

Fixes rdar://problem/51561208

Author: DougGregor

lib/Sema/TypeCheckCaptures.cpp

@@ -645,6 +645,12 @@ void TypeChecker::computeCaptures(AnyFunctionRef AFR) {
   }
 }
 
+static bool isLazy(PatternBindingDecl *PBD) {
+  if (auto var = PBD->getSingleVar())
+    return var->getAttrs().hasAttribute<LazyAttr>();
+  return false;
+}
+
 void TypeChecker::checkPatternBindingCaptures(NominalTypeDecl *typeDecl) {
   auto &ctx = typeDecl->getASTContext();
 
@@ -669,7 +675,7 @@ void TypeChecker::checkPatternBindingCaptures(NominalTypeDecl *typeDecl) {
                               /*ObjC=*/false);
       init->walk(finder);
 
-      if (finder.getDynamicSelfCaptureLoc().isValid()) {
+      if (finder.getDynamicSelfCaptureLoc().isValid() && !isLazy(PBD)) {
         ctx.Diags.diagnose(finder.getDynamicSelfCaptureLoc(),
                            diag::dynamic_self_stored_property_init);
       }

test/type/self.swift

@@ -253,4 +253,13 @@ class SelfStoredPropertyInit {
   static func myValue() -> Int { return 123 }
 
   var value = Self.myValue() // expected-error {{covariant 'Self' type cannot be referenced from a stored property initializer}}
-}
+}
+
+// rdar://problem/55273931 - erroneously rejecting 'Self' in lazy initializer
+class Foo {
+  static var value: Int = 17
+
+  lazy var doubledValue: Int = {
+    Self.value * 2
+  }()
+}