swift

8846f4e6 - [SE-0364] Handle retroactive conformance for types and protocols from underlying modules.

[SE-0364] Handle retroactive conformance for types and protocols from underlying modules. SE-0364 was implemented to discourage "retroactive" conformances that might conflict with conformances that could be introduced by other modules in the future. These diagnostics should not apply to conformances that involve types and protocols imported from the underlying clang module of a Swift module since the two modules are assumed to be developed in tandem by the same owners, despite technically being separate modules from the perspective of the compiler. The diagnostics implemented in https://github.com/apple/swift/pull/36068 were designed to take underlying clang modules into account. However, the implementation assumed that `ModuleDecl::getUnderlyingModuleIfOverlay()` would behave as expected when called on the Swift module being compiled. Unfortunately, it would always return `nullptr` and thus conformances involving the underlying clang module are being diagnosed unexpectedly. The fix is to make `ModuleDecl::getUnderlyingModuleIfOverlay()` behave as expected when it is made up of `SourceFile`s. Resolves rdar://121478556