InheritedProtocolConformance: Return the conforming type's DeclContext

[jckarter]

Not the superclass's. This can cause mapping types out of context to not
resolve archetypes, because the DeclContexts won't match, e.g:

internal class _IteratorBox<Base: IteratorProtocol> :
_AnyIteratorBoxBase<Base.Element> { internal override func next() ->
Base.Element? }

Here, _IteratorBox's associated type Element for IteratorProtocol is
Base.Element, but the conformance comes from the superclass,
_AnyIteratorBoxBase. This meant that Base.Element couldn't be mapped to
an interface type, which should be:

(dependent-member
(generic-type-parameter depth=0 index=0) member=Element)

The old behavior also broke inherited conformances across serialization boundaries, because subclasses would fail to inherit conformances a deserialized class inherited itself from a base class (SR-1158).

[jckarter]

@DougGregor Does this look reasonable for 2.2? There's something more subtle going on that I don't fully understand, and wouldn't want to mess with in the scope of 2.2. @bitjammer made this change to master as b0d6341 for other reasons, and it also gets deserialization working in the case of SR-1158.

[jckarter]

@swift-ci Please test

[jckarter]

@tkremenek Can we take this for 2.2 (contingent on tests passing and @DougGregor's approval)?

[tkremenek]

@jckarter Yes we can take it, assuming review and testing passes.

[tkremenek]

Test fails:

Testing: 0 .. 10.. 20
FAIL: Swift :: SILOptimizer/specialize_inherited.sil (1876 of 7874)
******************** TEST 'Swift :: SILOptimizer/specialize_inherited.sil' FAILED ********************
Script:
--
/home/buildnode/jenkins/workspace/swift-PR-Linux@2/buildbot_linux/swift-linux-x86_64/bin/sil-opt -target x86_64-unknown-linux-gnu  -module-cache-path '/tmp/swift-testsuite-clang-module-cacheH7m5_2' -enable-sil-verify-all /home/buildnode/jenkins/workspace/swift-PR-Linux@2/swift/test/SILOptimizer/specialize_inherited.sil -generic-specializer -module-name inherit | FileCheck /home/buildnode/jenkins/workspace/swift-PR-Linux@2/swift/test/SILOptimizer/specialize_inherited.sil
--
Exit Code: 1

Command Output (stderr):
--
/home/buildnode/jenkins/workspace/swift-PR-Linux@2/swift/test/SILOptimizer/specialize_inherited.sil:39:17: error: expected string not found in input
// CHECK-LABEL: @_TTSg5C7inherit8MMStringCS_8MMObjects8HashableS__GSQCS_6MMFont___callee : $@convention(method) (@in MMString, Int, @owned MMStorage<MMString, ImplicitlyUnwrappedOptional<MMFont>>) -> Bool {
                ^
<stdin>:31:1: note: scanning from here
// %0 // user: %4
^
<stdin>:47:23: note: possible intended match here
sil shared [noinline] @_TTSg5C7inherit8MMStringS0_s8HashableS__GSQCS_6MMFont___callee : $@convention(method) (@in MMString, Int, @owned MMStorage<MMString, ImplicitlyUnwrappedOptional<MMFont>>) -> Bool {
                      ^

--

[tkremenek]

@jckarter Is this just a test case that needs to be updated, or something more?

[jckarter]

@tkremenek Yeah, the test case can just be updated. I'll take care of it and start the tests again.

[tkremenek]

FYI, @DougGregor signed off offline on this change.

[jckarter]

@swift-ci Please test