Sema: Prioritize @_exported imports over local non-public imports

Update the logic selecting the most restrictive import for a given
reference to account for @_exported imports from the local module. We
should always prioritize @_exported imports from the local module over
more restrictive same file imports. Only if an import from the same file
is also public we prefer it as it's more useful for diagnostics and
generally recommended to locally declare dependencies.

Also update the test that was meant to check this configuration to apply
two different variations, one for a module local @_exported and one
relying on the underlying clang module.

rdar://140924031

Author: xymus

lib/AST/Module.cpp

@@ -2944,6 +2944,16 @@ SourceFile::getImportAccessLevel(const ModuleDecl *targetModule) const {
       restrictiveImport = import;
     }
   }
+
+  // Reexports from the local module take precedence over non-public imports
+  // and lift all access-level restrictions. We still prioritize file local
+  // public imports as diagnostics will have an import to point to and
+  // they are recommended over indirect imports.
+  if ((!restrictiveImport.has_value() ||
+       restrictiveImport->accessLevel < AccessLevel::Public) &&
+     imports.isImportedBy(targetModule, getParentModule()))
+    return std::nullopt;
+
   return restrictiveImport;
 }
 

test/Sema/access-level-import-exported.swift

@@ -0,0 +1,55 @@
+/// Test that an @_exported import is preferred to local non-public imports.
+// RUN: %empty-directory(%t)
+// RUN: split-file --leading-lines %s %t
+
+/// Build the libraries.
+// RUN: %target-swift-frontend -emit-module %t/TargetLib.swift -o %t
+// RUN: %target-swift-frontend -emit-module %t/IndirectLib.swift -I %t -o %t
+
+/// Check acceptable client configurations to access TargetLib publicly.
+// RUN: %target-swift-frontend -typecheck -verify -I %t \
+// RUN:   %t/Client_ImportDirect.swift
+// RUN: %target-swift-frontend -typecheck -verify -I %t \
+// RUN:   %t/Client_ImportDirect.swift %t/Client_FileReexport.swift
+// RUN: %target-swift-frontend -typecheck -verify -I %t \
+// RUN:   %t/Client_ImportDirectAsPrivate.swift %t/Client_FileReexport.swift
+// RUN: %target-swift-frontend -typecheck -verify -I %t \
+// RUN:   %t/Client_ImportIndirectModule.swift
+// RUN: %target-swift-frontend -typecheck -verify -I %t \
+// RUN:   %t/Client_ImportIndirectModule.swift %t/Client_FileReexport.swift
+// RUN: %target-swift-frontend -typecheck -verify -I %t \
+// RUN:   %t/Client_ImportIndirectModuleAsPrivate.swift %t/Client_FileReexport.swift
+// RUN: %target-swift-frontend -typecheck -verify -I %t \
+// RUN:   %t/Client_ImportIndirectLocal.swift %t/Client_FileReexport.swift
+
+//--- TargetLib.swift
+public struct TargetType {
+    public init() {}
+}
+
+//--- IndirectLib.swift
+@_exported import TargetLib
+
+//--- Client_FileReexport.swift
+@_exported public import TargetLib
+
+//--- Client_ImportDirect.swift
+public import TargetLib
+public func user(t: TargetType) {}
+
+//--- Client_ImportDirectAsPrivate.swift
+fileprivate import TargetLib
+public func user(t: TargetType) {}
+// Unrestricted as it's @_exported elsewhere in the module
+
+//--- Client_ImportIndirectModule.swift
+public import IndirectLib
+public func user(t: TargetType) {}
+
+//--- Client_ImportIndirectModuleAsPrivate.swift
+fileprivate import IndirectLib
+public func user(t: TargetType) {}
+// Unrestricted as it's @_exported elsewhere in the module
+
+//--- Client_ImportIndirectLocal.swift
+public func user(t: TargetType) {}

test/Sema/authoritative-import-priority.swift

@@ -17,11 +17,21 @@
 /// Client testing order of preference for more levels of imports.
 // RUN: %target-swift-frontend -typecheck -I %t \
 // RUN:   %t/ExportedClient_FileExported.swift %t/ExportedClient_FileA.swift \
-// RUN:   %t/ExportedClient_FileB.swift %t/ExportedClient_FileC.swift %t/ExportedClient_FileD.swift \
+// RUN:   %t/ExportedClient_FileB.swift %t/ExportedClient_FileC.swift \
+// RUN:   %t/ExportedClient_FileD_via_underlying.swift \
 // RUN:   -import-underlying-module -module-name ExportedClient \
 // RUN:   -enable-upcoming-feature InternalImportsByDefault \
 // RUN:   -Rmodule-api-import -verify
 
+// Same without the underlying clang module.
+// RUN: %target-swift-frontend -typecheck -I %t \
+// RUN:   %t/ExportedClient_FileExported.swift %t/ExportedClient_FileA.swift \
+// RUN:   %t/ExportedClient_FileB.swift %t/ExportedClient_FileC.swift \
+// RUN:   %t/ExportedClient_FileD_via_exported.swift \
+// RUN:   -module-name ExportedClient \
+// RUN:   -enable-upcoming-feature InternalImportsByDefault \
+// RUN:   -Rmodule-api-import -verify
+
 /// Client testing -public-module-name ordering.
 // RUN: %target-swift-frontend -typecheck -I %t \
 // RUN:   %t/SwiftLibClient_FileA.swift %t/SwiftLibClient_FileB.swift \
@@ -156,11 +166,16 @@ public import NotLib
 public func useTypesC(a: ExportedType) {}
 // expected-remark @-1 {{struct 'ExportedType' is imported via 'NotLib', which reexports definition from 'LibCore'}}
 
-//--- ExportedClient_FileD.swift
-/// Last used the module-wide @_exported import.
+//--- ExportedClient_FileD_via_underlying.swift
+/// Then use the import of the underling clang module.
 public func useTypesD(a: ExportedType) {}
 // expected-remark @-1 {{struct 'ExportedType' is imported via 'ExportedClient', which reexports definition from 'LibCore'}}
 
+//--- ExportedClient_FileD_via_exported.swift
+/// Finally use the @_exported import from the local module.
+public func useTypesD(a: ExportedType) {}
+// It would be nice to have a remark even without an import to point to.
+
 
 //--- SwiftLibClient_FileA.swift
 /// Prefer the import matching public-module-name.