AST: Diagnose references to non-public declarations from default arguments of inlinable local functions

Part of <https://bugs.swift.org/browse/SR-12404>.

Author: slavapestov

lib/AST/DeclContext.cpp

@@ -348,6 +348,13 @@ swift::FragileFunctionKindRequest::evaluate(Evaluator &evaluator,
       auto *VD = cast<ValueDecl>(dc->getAsDecl());
       assert(VD->hasParameterList());
 
+      if (VD->getDeclContext()->isLocalContext()) {
+        auto kind = VD->getDeclContext()->getFragileFunctionKind();
+        if (kind.kind != FragileFunctionKind::None)
+          return {FragileFunctionKind::DefaultArgument,
+                  kind.allowUsableFromInline};
+      }
+
       auto effectiveAccess =
         VD->getFormalAccessScope(/*useDC=*/nullptr,
                                  /*treatUsableFromInlineAsPublic=*/true);

test/attr/attr_inlinable.swift

@@ -9,11 +9,11 @@
 // expected-warning@-1 {{'@inlinable' declaration is already '@usableFromInline'}}
 
 private func privateFunction() {}
-// expected-note@-1{{global function 'privateFunction()' is not '@usableFromInline' or public}}
+// expected-note@-1 2{{global function 'privateFunction()' is not '@usableFromInline' or public}}
 fileprivate func fileprivateFunction() {}
 // expected-note@-1{{global function 'fileprivateFunction()' is not '@usableFromInline' or public}}
 func internalFunction() {}
-// expected-note@-1{{global function 'internalFunction()' is not '@usableFromInline' or public}}
+// expected-note@-1 2{{global function 'internalFunction()' is not '@usableFromInline' or public}}
 @usableFromInline func versionedFunction() {}
 public func publicFunction() {}
 
@@ -306,3 +306,16 @@ public struct PrivateInlinableCrash {
     value ? "YES" : "NO"
   }
 }
+
+// https://bugs.swift.org/browse/SR-12404
+@inlinable public func inlinableOuterFunction() {
+  func innerFunction1(x: () = privateFunction()) {}
+  // expected-error@-1 {{global function 'privateFunction()' is private and cannot be referenced from a default argument value}}
+
+  func innerFunction2(x: () = internalFunction()) {}
+  // expected-error@-1 {{global function 'internalFunction()' is internal and cannot be referenced from a default argument value}}
+
+  func innerFunction3(x: () = versionedFunction()) {}
+
+  func innerFunction4(x: () = publicFunction()) {}
+}