[Sema] Report conflicts between @_exported and non-public imports

@_exported can only be used on public imports, it's incompatible with
any other access level. This implementation doesn't take into account
Swift 6 mode.

Author: xymus

include/swift/AST/DiagnosticsSema.def

@@ -2057,6 +2057,9 @@ ERROR(access_level_on_import_unsupported, none,
       "only 'public', 'package', 'internal', 'fileprivate' and 'private' "
       "are unsupported",
       (DeclAttribute))
+ERROR(access_level_conflict_with_exported,none,
+      "'%0' is incompatible with %1; it can only be applied to public imports",
+      (DeclAttribute, DeclAttribute))
 
 // Opaque return types
 ERROR(opaque_type_invalid_constraint,none,

lib/Sema/TypeCheckAttr.cpp

@@ -970,6 +970,14 @@ bool AttributeChecker::visitAbstractAccessControlAttr(
                             attr);
       return true;
     }
+
+    if (attr->getAccess() != AccessLevel::Public) {
+      if (auto exportedAttr = D->getAttrs().getAttribute<ExportedAttr>()) {
+        diagnoseAndRemoveAttr(attr, diag::access_level_conflict_with_exported,
+                              exportedAttr, attr);
+        return true;
+      }
+    }
   }
 
   return false;

test/Sema/conflicting-import-restrictions.swift

@@ -35,3 +35,35 @@
 
 //--- SPIOnly_IOI_Exported.swift
 @_spiOnly @_implementationOnly @_exported import Lib // expected-error {{module 'Lib' cannot be both exported and implementation-only}}
+
+/// Access levels on imports
+// RUN: %target-swift-frontend -typecheck %t/Public_Exported.swift -I %t -verify \
+// RUN:   -experimental-spi-only-imports -verify \
+// RUN:   -enable-experimental-feature AccessLevelOnImport
+// RUN: %target-swift-frontend -typecheck %t/Package_Exported.swift -I %t -verify \
+// RUN:   -experimental-spi-only-imports -verify \
+// RUN:   -enable-experimental-feature AccessLevelOnImport
+// RUN: %target-swift-frontend -typecheck %t/Internal_Exported.swift -I %t -verify \
+// RUN:   -experimental-spi-only-imports -verify \
+// RUN:   -enable-experimental-feature AccessLevelOnImport
+// RUN: %target-swift-frontend -typecheck %t/Fileprivate_Exported.swift -I %t -verify \
+// RUN:   -experimental-spi-only-imports -verify \
+// RUN:   -enable-experimental-feature AccessLevelOnImport
+// RUN: %target-swift-frontend -typecheck %t/Private_Exported.swift -I %t -verify \
+// RUN:   -experimental-spi-only-imports -verify \
+// RUN:   -enable-experimental-feature AccessLevelOnImport
+
+//--- Public_Exported.swift
+@_exported public import Lib
+
+//--- Package_Exported.swift
+@_exported package import Lib // expected-error {{'@_exported' is incompatible with 'package'; it can only be applied to public imports}}
+
+//--- Internal_Exported.swift
+@_exported internal import Lib // expected-error {{'@_exported' is incompatible with 'internal'; it can only be applied to public imports}}
+
+//--- Fileprivate_Exported.swift
+@_exported fileprivate import Lib // expected-error {{'@_exported' is incompatible with 'fileprivate'; it can only be applied to public imports}}
+
+//--- Private_Exported.swift
+@_exported private import Lib // expected-error {{'@_exported' is incompatible with 'private'; it can only be applied to public imports}}