relates to:

• SR-15586 import Foundation hides declarations (derived from C) in other modules on Linux.

Issue Description:

Introduction

This is just a reduced case of SR-15586, however, it indicates that this issue occurs on any platforms; of course, even on macOS.

How to reproduce

Files

<current directory>
+-  library.h
+-  library.swift
+-  MyClangModule
|    +-  module.modulemap
+- user.swift

Contents of files

library.h

// library.h
#ifndef LIBRARY_H__________
#define LIBRARY_H__________
typedef struct _MyCStruct {} MyCStruct;
#endif

library.swift

// library.swift
public typealias SwiftyName = MyCStruct
extension SwiftyName {
  public var foo: String { return "foo" }
  public var foobar: String { return foo + "bar" }
}

MyClangModule/module.modulemap

// MyClangModule/module.modulemap
module MyClangModule {
  header "../library.h"
  export *
}

user.swift

// user.swift
// Unnecessary to `import MyClangModule` to induce the issue
import MySwiftLibrary
let instance = SwiftyName()
print(instance.foo)
print(instance.foobar)

Steps

Compile library

swiftc library.swift \
  -import-objc-header library.h \
  -emit-module -emit-library -module-name MySwiftLibrary

Compile executable

swiftc user.swift -o ./user -I. -L. -lMySwiftLibrary

Result

user.swift:5:16: error: value of type 'SwiftyName' (aka '_MyCStruct') has no member 'foo'
print(instance.foo)
      ~~~~~~~~ ^~~
user.swift:6:16: error: value of type 'SwiftyName' (aka '_MyCStruct') has no member 'foobar'
print(instance.foobar)
      ~~~~~~~~ ^~~~~~

Discussion

The reason why this issue occurs is the same with SR-15586.
As described in the comment in "user.swift", `import MyClangModule` is not necessary to induce this error.
Only the existence of "MyClangModule" directory in one of the search paths causes the error.
Removing "MyClangModule" directory makes it possible to compile "user.swift".

ClangImporter has one unique instance of clang::Parser, clang::Preprocessor, and so on, but it has separate tables for clang::Module s and for declarations derived from simple C headers.
Such implementation brings about this kind of problems.