Sema: Diagnose DynamicSelfType usages inside stored property initializers

A stored property initializer does not have access to either the
'self' instance or the 'Self' type, so SILGen would just crash
when emitting one containing a reference to 'Self'.

With some work on SILGen we could probably allow this, but for
now let's diagnose instead of crashing.

Fixes <rdar://problem/51561208>, <https://bugs.swift.org/browse/SR-10969>.

Author: slavapestov

include/swift/AST/DiagnosticsSema.def

@@ -2595,6 +2595,8 @@ ERROR(dynamic_self_invalid_subscript,none,
       "covariant 'Self' can only appear at the top level of subscript element type", ())
 ERROR(dynamic_self_invalid_method,none,
       "covariant 'Self' can only appear at the top level of method result type", ())
+ERROR(dynamic_self_stored_property_init,none,
+      "covariant 'Self' type cannot be referenced from a stored property initializer", ())
 
 //------------------------------------------------------------------------------
 // MARK: Type Check Attributes

lib/Sema/TypeCheckCaptures.cpp

@@ -80,6 +80,10 @@ class FindCapturedVars : public ASTWalker {
     return GenericParamCaptureLoc;
   }
 
+  SourceLoc getDynamicSelfCaptureLoc() const {
+    return DynamicSelfCaptureLoc;
+  }
+
   /// Check if the type of an expression references any generic
   /// type parameters, or the dynamic Self type.
   ///
@@ -698,6 +702,11 @@ void TypeChecker::checkPatternBindingCaptures(NominalTypeDecl *typeDecl) {
                               /*ObjC=*/false);
       init->walk(finder);
 
+      if (finder.getDynamicSelfCaptureLoc().isValid()) {
+        diagnose(finder.getDynamicSelfCaptureLoc(),
+                 diag::dynamic_self_stored_property_init);
+      }
+
       auto captures = finder.getCaptureInfo();
       PBD->setCaptureInfo(i, captures);
     }

test/type/self.swift

@@ -144,6 +144,7 @@ class C {
 
   static func staticFunc() -> Self {}
   let stored: Self = Self.staticFunc() // expected-error {{stored property cannot have covariant 'Self' type}}
+  // expected-error@-1 {{covariant 'Self' type cannot be referenced from a stored property initializer}}
 
   var prop: Self { // expected-error {{mutable property cannot have covariant 'Self' type}}
     get {
@@ -247,3 +248,9 @@ enum E {
     return .e
   }
 }
+
+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}}
+}