Re-exporting a module in a modulemap does not work

[Steelskin]

Description

Consider the following modulemap file:

module Interop {
    header "interop.h"
    requires cplusplus
}

module Container {
    export Interop
}

The modulemap documentation states that:

An export-declaration specifies which imported modules will automatically be re-exported as part of a given module’s API.

However, when importing Container in Swift, the symbols defined in interop.h are not available.

Reproduction

// ImportInterop.swift
import Container

var result = InteropClass();
result.property = true;

// interop/interop.h
#pragma once

class InteropClass {
public:
    bool property = false;
};

// interop/module.modulemap
module Interop {
    header "interop.h"
    requires cplusplus
}

module Container {
    export Interop
}

Build command:

swiftc -v -module-cache-path clang-module-cache/ -Xfrontend -cxx-interoperability-mode=default -I interop/ -c ImportInterop.swift -o ImportInterop.o

Expected behavior

The above code should compile properly. InteropClass should be a valid symbol.

Environment

Swift 6.1-dev (recent-ish trunk build)

Additional information

No response

[Steelskin]

This comment on the LLVM bug leads me to believe we have been holding the modulemap for ucrt wrong:

Clang header modules can only export entities that they have a declaration for. export in a module map just allows declarations to be exported, it doesn't add any imports itself. The Container module needs a header that includes interop.h.

export does not actually import the declarations, which is what we thought was happening. I am not certain what the best way to solve the issue would be. Maybe the swift compiler should be making sure all of the symbols are re-exported since this is how we are using module maps for C++/Swift interop.