Open Radar

 
Summary:
I constraint a type parameter to conform to a protocol, T. That protocol conforms to Equatable as well as adding its own constraints. I then extend Character correctly to conform to T. Now I can’t link, because of duplicate symbols declaring Equatable conformance.


Steps to Reproduce:
1. Compile this code (NB: you can’t just run it in a script, it has to actually compile & [fail to] link):

protocol T : Equatable {}
extension Character : T {}


Expected Results:
I expected this would compile and link just fine.


Actual Results:
It compiles but fails to link:

duplicate symbol __TTWOSs9CharacterSs9EquatableFS0_oi2eeUS0___fMQPS0_FTS1_S1__Sb in:
    /Users/rob/Library/Developer/Xcode/DerivedData/boom-aaolihuubdazajalrhptcajlaiaf/Build/Intermediates/boom.build/Debug/boom.build/Objects-normal/x86_64/main.o
    /Applications/Xcode-Beta.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/swift_static/macosx/libswiftCore.a(Swift.o)
duplicate symbol __TWPOSs9CharacterSs9Equatable in:
    /Users/rob/Library/Developer/Xcode/DerivedData/boom-aaolihuubdazajalrhptcajlaiaf/Build/Intermediates/boom.build/Debug/boom.build/Objects-normal/x86_64/main.o
    /Applications/Xcode-Beta.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/swift_static/macosx/libswiftCore.a(Swift.o)
ld: 2 duplicate symbols for architecture x86_64


Regression:
Doesn’t happen if you just run this as a script, presumably because it’s not generating linker symbols quite the same way.


Notes:
This also applies when the protocol conformance is declared elsewhere in your own project, e.g. if I make a struct S and conform it to Equatable explicitly and then also conform it to my protocol T : Equatable.