Handle top-level vars' and closure params' access specially.

Parameters are normally given 'private' access, because they can only
be referred to within the body of the owning function. However,
single-expression closures allow a parameter to appear in a constraint
system in the containing context. Mark closure parameters as
'fileprivate' instead.

Similarly, 'private' at the top level is normally equivalent to
'fileprivate', but not for a decl that appears within top-level
imperative code, which has a TopLevelCodeDecl context. This currently
only happens for bindings in a top-level 'guard' statement; mark
these variables and constants as 'fileprivate' as well.

More progress on SE-0025 ('private' and 'fileprivate').

Author: jrose-apple

lib/Sema/TypeCheckDecl.cpp

@@ -1354,10 +1354,22 @@ void TypeChecker::computeAccessibility(ValueDecl *D) {
   if (!D->hasAccessibility()) {
     DeclContext *DC = D->getDeclContext();
     switch (DC->getContextKind()) {
-    case DeclContextKind::SerializedLocal:
+    case DeclContextKind::TopLevelCodeDecl:
+      // Variables declared in a top-level 'guard' statement can be accessed in
+      // later top-level code.
+      D->setAccessibility(Accessibility::FilePrivate);
+      break;
     case DeclContextKind::AbstractClosureExpr:
+      if (isa<ParamDecl>(D)) {
+        // Closure parameters may need to be accessible to the enclosing
+        // context, for single-expression closures.
+        D->setAccessibility(Accessibility::FilePrivate);
+      } else {
+        D->setAccessibility(Accessibility::Private);
+      }
+      break;
+    case DeclContextKind::SerializedLocal:
     case DeclContextKind::Initializer:
-    case DeclContextKind::TopLevelCodeDecl:
     case DeclContextKind::AbstractFunctionDecl:
     case DeclContextKind::SubscriptDecl:
       D->setAccessibility(Accessibility::Private);

test/expr/capture/top-level-guard.swift

@@ -39,7 +39,7 @@ let closureCapture: () -> Void = { [x] in
 }
 
 // CHECK-LABEL: (defer_stmt
-// CHECK-NEXT: (func_decl implicit "$defer()" type='() -> ()' access=private captures=(x<direct><noescape>)
+// CHECK-NEXT: (func_decl implicit "$defer()" type='() -> ()' access=fileprivate captures=(x<direct><noescape>)
 defer {
   _ = x
 }