Open Radar

 
(note: sample project is at http://cloud.ashfurrow.com/2I2V193C3l0z )

Summary:
Ran into this problem. I have created a class that uses a generic, and I have an extension to that class *in another file* (this is really important). In my extension, I have a function that takes an argument of the type T, the generic type of my class. Then in my unit tests, I import the module created by my main target and create an instance of my class. Trying to call my extension's method yields a linker error.

Steps to Reproduce:
1. New Xcode project ("Single View" template)
2. Create new Swift file "Test.swift" in the main target and give it the following contents:

import Foundation

public class A<T> {
    var t: T?
    public init () {
        
    }
}

3.Create another new file in the main target called "Test+Extension.swift" and give it the following contents:

import Foundation

extension A {
    public func doSomething (c: T) -> String {
        return "hi"
    }
}

4. In your unit testing target, add an import statement at the top with the name of your main target. 
5. In a unit test, create an instance of A<Int> and then try and call doSomething(2)
6. Run unit tests

Expected Results:
doSomething(2) returns "hi"

Actual Results:
Following linker error: 

Ld /Users/ash/Library/Developer/Xcode/DerivedData/BugTest-gpaacovdzzcczecuomisszsbnmon/Build/Products/Debug-iphonesimulator/BugTest.app/BugTest normal x86_64
    cd /Users/ash/Desktop/BugTest
    export IPHONEOS_DEPLOYMENT_TARGET=8.0
    export PATH="/Applications/Xcode6-Beta6.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/usr/bin:/Applications/Xcode6-Beta6.app/Contents/Developer/usr/bin:/usr/bin:/bin:/usr/sbin:/sbin"
    /Applications/Xcode6-Beta6.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang -arch x86_64 -isysroot /Applications/Xcode6-Beta6.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator8.0.sdk -L/Users/ash/Library/Developer/Xcode/DerivedData/BugTest-gpaacovdzzcczecuomisszsbnmon/Build/Products/Debug-iphonesimulator -F/Users/ash/Library/Developer/Xcode/DerivedData/BugTest-gpaacovdzzcczecuomisszsbnmon/Build/Products/Debug-iphonesimulator -filelist /Users/ash/Library/Developer/Xcode/DerivedData/BugTest-gpaacovdzzcczecuomisszsbnmon/Build/Intermediates/BugTest.build/Debug-iphonesimulator/BugTest.build/Objects-normal/x86_64/BugTest.LinkFileList -Xlinker -rpath -Xlinker @executable_path/Frameworks -Xlinker -objc_abi_version -Xlinker 2 -lswiftCore -L/Applications/Xcode6-Beta6.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/swift/iphonesimulator -Xlinker -add_ast_path -Xlinker /Users/ash/Library/Developer/Xcode/DerivedData/BugTest-gpaacovdzzcczecuomisszsbnmon/Build/Intermediates/BugTest.build/Debug-iphonesimulator/BugTest.build/Objects-normal/x86_64/BugTest.swiftmodule -mios-simulator-version-min=8.0 -Xlinker -dependency_info -Xlinker /Users/ash/Library/Developer/Xcode/DerivedData/BugTest-gpaacovdzzcczecuomisszsbnmon/Build/Intermediates/BugTest.build/Debug-iphonesimulator/BugTest.build/Objects-normal/x86_64/BugTest_dependency_info.dat -o /Users/ash/Library/Developer/Xcode/DerivedData/BugTest-gpaacovdzzcczecuomisszsbnmon/Build/Products/Debug-iphonesimulator/BugTest.app/BugTest

Undefined symbols for architecture x86_64:
  "_swift_allocateGenericClassMetadata", referenced from:
      _create_generic_metadata in Test.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)