Sema: Fix corner case with @_fixed_layout diagnostic

We allow a member of a type to be more accessible than the type
itself. In this case, the broader accessibility is ignored, and
the effective access of the member is constrained by the
accessibility of its parent type.

Author: slavapestov

lib/Sema/TypeCheckAttr.cpp

@@ -1773,14 +1773,19 @@ void AttributeChecker::visitSpecializeAttr(SpecializeAttr *attr) {
   attr->setRequirements(DC->getASTContext(), convertedRequirements);
 }
 
+static Accessibility getAccessForDiagnostics(const ValueDecl *D) {
+  return std::min(D->getFormalAccess(),
+                  D->getEffectiveAccess());
+}
+
 void AttributeChecker::visitFixedLayoutAttr(FixedLayoutAttr *attr) {
   auto *VD = cast<ValueDecl>(D);
 
   if (VD->getEffectiveAccess() < Accessibility::Public) {
     TC.diagnose(attr->getLocation(),
                 diag::fixed_layout_attr_on_internal_type,
                 VD->getName(),
-                VD->getFormalAccess())
+                getAccessForDiagnostics(VD))
         .fixItRemove(attr->getRangeWithAt());
     attr->setInvalid();
   }
@@ -1803,7 +1808,7 @@ void AttributeChecker::visitVersionedAttr(VersionedAttr *attr) {
     TC.diagnose(attr->getLocation(),
                 diag::versioned_attr_with_explicit_accessibility,
                 VD->getName(),
-                VD->getFormalAccess())
+                getAccessForDiagnostics(VD))
         .fixItRemove(attr->getRangeWithAt());
     attr->setInvalid();
     return;

test/attr/attr_fixed_layout.swift

@@ -50,9 +50,13 @@ struct Rectangle {
 // Diagnostics
 //
 
-@_fixed_layout struct InternalStruct {}
+@_fixed_layout struct InternalStruct {
 // expected-error@-1 {{'@_fixed_layout' attribute can only be applied to '@_versioned' or public declarations, but 'InternalStruct' is internal}}
 
+  @_fixed_layout public struct NestedStruct {}
+  // expected-error@-1 {{'@_fixed_layout' attribute can only be applied to '@_versioned' or public declarations, but 'NestedStruct' is internal}}
+}
+
 @_fixed_layout fileprivate struct FileprivateStruct {}
 // expected-error@-1 {{'@_fixed_layout' attribute can only be applied to '@_versioned' or public declarations, but 'FileprivateStruct' is fileprivate}}
 

validation-test/compiler_crashers/28660-false-encountered-error-in-diagnostic-text.swift renamed to validation-test/compiler_crashers_fixed/28660-false-encountered-error-in-diagnostic-text.swift

@@ -6,5 +6,5 @@
 // See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
 
 // REQUIRES: asserts
-// RUN: not --crash %target-swift-frontend %s -emit-ir
+// RUN: not %target-swift-frontend %s -emit-ir
 class C{@_fixed_layout public struct P