Oh, interesting. This is a Swift package, so 'Supports Text-Based InstallAPI' is not enabled. This happens because we set `-validate-tbd-against-ir=missing` in assertions-enabled builds. Can you switch from the swift-5.1-RELEASE toolchain to using Xcode 11 proper instead? That won't have this verifier enabled.
This happens because foo() is defined in the header, and is not a `static` function, which gives it external linkage and also embeds its definition inside this module. If you put the definition of `foo` in the .c file instead, this no longer errors.
@belkadan, should we be considering functions defined in a header as part of our module? I didn't even realize we compile those.