AST: Fix a problem with the protocol order #80213
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tshortli
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Mar 21, 2025
The last step in building a generic signature is to sort the requirements. Requirements are sorted by comparing their subject types. If two requirements have the same subject type, which can only happen with conformance requirements, we break the tie by comparing protocol declarations. We compare protocol declarations using TypeDecl::compare(), which is a shortlex order on the components of the fully qualified name of a protocol (eg, Swift.Sequence, etc.) While this order is part of the ABI, it has not been updated over the years for several important changes: - It did not handle module aliases; if we import a module via an alias, we should use the real module name to compare protocols, and not the aliased name. This produced inconsistent results if the same module was imported under different names, which can happen with module interface files that use module aliases. - It did not handle the -module-abi-name flag. Changing the ABI name of a module changes how we mangle protocol names, and the order should match the mangling. This change uses the correct module names for comparison to match what the mangler does. Of course this is an ABI break, however it should have limited impact since module aliases and -module-abi-name are not widely used. Fixes rdar://147441890.
If a protocol was moved from one module to another, we need to continue using the old module name when comparing it against other protocols.
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The last step in building a generic signature is to sort the requirements. Requirements are sorted by comparing their subject types. If two requirements have the same subject type, which can only happen with conformance requirements, we break the tie by comparing protocol declarations.
We compare protocol declarations using TypeDecl::compare(), which is a shortlex order on the components of the fully qualified name of a protocol (eg, Swift.Sequence, etc.)
Ultimately, the protocol order determines mangled symbol names, the order of witness table arguments in the calling convention, the layout of existentials, and so on. While this order is part of the ABI, it has not been updated over the years for several important changes:
It did not handle module aliases; if we import a module via an alias, we should use the real module name to compare protocols, and not the aliased name. This produced inconsistent results if the same module was imported under different names, which can happen with module interface files that use module aliases.
It did not handle the
-module-abi-nameflag. Changing the ABI name of a module changes how we mangle protocol names, and the order should match the mangling.It did not use the original module name specified in the
@_originallyDefinedInflag, so moving a protocol between modules would change its position in the order.This PR fixes the above problems. Of course this is an ABI break, however it should have limited impact since module aliases and -module-abi-name are not widely used.
Fixes rdar://147441890.