Allan Shortlidge 8846f4e691 [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

2024-02-01 10:43:53 -08:00

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