Error protocol constraint

[minuscorp]

This PR adds the Error protocol constraint to throwing types but fails in (among other things surely) serialize variables that come from protocols that bridge from ObjC variables it seems (like Error protocol) other protocols seems to work ok, both own ones like CustomStringConvertible or anyone from Foundation or another modules that can be imported. I can't. trace more the error but not the root of itself.

frame #3: 0x0000000101a44d4e swift-frontend`swift::ModuleFileSharedCore::fatal(error=<unavailable>) at ModuleFileSharedCore.cpp:453:3
    frame #4: 0x000000010193cf82 swift-frontend`swift::ModuleFile::fatal(this=0x000000012c00fa00, error=<unavailable>) at Deserialization.cpp:192:3
    frame #5: 0x000000010193d329 swift-frontend`swift::ModuleFile::getDecl(this=0x000000012c00fa00, DID=(Value = 2353642078208)) at Deserialization.cpp:2327:5
    frame #6: 0x0000000101949fe3 swift-frontend`swift::ModuleFile::configureStorage(this=0x000000012c00fa00, decl=0x0000000134864388, rawOpaqueReadOwnership='\0', rawReadImplKind='\x01', rawWriteImplKind='\0', rawReadWriteImplKind='\0', rawIDs=0x00007ffeefbf7690) at Deserialization.cpp:2251:52
    frame #7: 0x00000001019557cf swift-frontend`swift::DeclDeserializer::deserializeVar(this=0x00007ffeefbf7f10, scratch=ArrayRef<unsigned long long> @ 0x00007ffeefbf7740, blobData=(Data = 0x0000000000000000, Length = 0)) at Deserialization.cpp:2847:8
    frame #8: 0x000000010194ae36 swift-frontend`swift::DeclDeserializer::getDeclCheckedImpl(this=0x00007ffeefbf7f10, matchAttributes=function_ref<bool (swift::DeclAttributes)> @ 0x00007ffeefbf7e30)>) at Deserialization.cpp:4565:3
    frame #9: 0x000000010193e56a swift-frontend`swift::ModuleFile::getDeclChecked(this=0x000000012c00fa00, DID=(Value = 48704929136640), matchAttributes=function_ref<bool (swift::DeclAttributes)> @ 0x00007ffeefbf7fb0)>) at Deserialization.cpp:4051:45
    frame #10: 0x000000010193da45 swift-frontend`swift::ModuleFile::readPattern(this=0x000000012c00fa00, owningDC=0x0000000134863fc8) at Deserialization.cpp:354:25
    frame #11: 0x000000010193dc4c swift-frontend`swift::ModuleFile::readPattern(this=0x000000012c00fa00, owningDC=0x0000000134863fc8) at Deserialization.cpp:380:38
    frame #12: 0x0000000101956ca4 swift-frontend`swift::DeclDeserializer::deserializePatternBinding(this=0x00007ffeefbf9160, scratch=ArrayRef<unsigned long long> @ 0x00007ffeefbf89c0, blobData=(Data = 0x0000000000000000, Length = 0)) at Deserialization.cpp:3315:25
    frame #13: 0x000000010194b25f swift-frontend`swift::DeclDeserializer::getDeclCheckedImpl(this=0x00007ffeefbf9160, matchAttributes=function_ref<bool (swift::DeclAttributes)> @ 0x00007ffeefbf9080)>) at Deserialization.cpp:4570:3
    frame #14: 0x000000010193e56a swift-frontend`swift::ModuleFile::getDeclChecked(this=0x000000012c00fa00, DID=(Value = 48765058678784), matchAttributes=function_ref<bool (swift::DeclAttributes)> @ 0x00007ffeefbf9200)>) at Deserialization.cpp:4051:45
    frame #15: 0x000000010196156b swift-frontend`swift::ModuleFile::loadAllMembers(this=0x000000012c00fa00, container=0x0000000134863fd0, contextData=56099726) at Deserialization.cpp:5946:29

Last readable context:

(var_decl "_domain" type='String' interface type='String' readImpl=getter writeImpl=setter readWriteImpl=modify_coroutine)

2.	While evaluating request TypeCheckSourceFileRequest(source_file "/Users/minuscorp/Documents/swift-source/swift/test/Parse/errors.swift")
3.	While type-checking 'SomeError' (at /Users/minuscorp/Documents/swift-source/swift/test/Parse/errors.swift:3:1)
4.	While type-checking protocol conformance to 'Error' (in module 'Swift') for type 'SomeError' (declared at [/Users/minuscorp/Documents/swift-source/swift/test/Parse/errors.swift:3:1 - line:3:26] RangeText="struct SomeError: Error {")
5.	While loading members for 'Error' (in module 'Swift')
6.	While deserializing decl @ 58232271 (PATTERN_BINDING_DECL) in 'Swift'
7.	While deserializing '_domain' (VarDecl @ 58230816) in 'Swift'
8.	    ...decl is named '_domain'

[minuscorp]

there's no way to use getNeverType here, is always null

[minuscorp]

this fails always in tests

[DougGregor]

You cannot get the canonical type from a null Type. Try:

CanType throwsTy;
if (auto funcThrowsTy = funcTy->getThrowsType())
  throwsTy = funcThrowsTy->getCanonicalType(genericSig);

[minuscorp]

What happens with all these traces, is Never not usable to be injected as a default throwsType, even if the function doesn't throw:

swift-frontend           0x00000001039995ac swift::DeclContext::getAsDecl() + 28
6  swift-frontend           0x00000001039ebbd5 swift::DeclContext::getAsDecl() const + 21
7  swift-frontend           0x0000000103c751d5 swift::DeclContext::isModuleContext() const + 21
8  swift-frontend           0x0000000103d8d3dd swift::DeclContext::getParentModule() const + 29
9  swift-frontend           0x0000000103d8d3a5 swift::DeclContext::getASTContext() const + 21
10 swift-frontend           0x000000010397bfae swift::Decl::getASTContext() const + 62
11 swift-frontend           0x0000000103c55f34 computeNominalType(swift::NominalTypeDecl*, DeclTypeKind) + 36
12 swift-frontend           0x0000000103c5576d swift::NominalTypeDecl::getDeclaredInterfaceType() const + 93
13 swift-frontend           0x00000001039755bf swift::ASTContext::getNeverType() const + 63