0x0001015c51b0 by thread T1: #0 closure #1 in assign.swift:5 (assign:x86_64+0x1000019cc) #1 thunk for @callee_owned () $-> () assign.swift (assign:x86_64+0x100001a4c) #2 $__tsan$::invoke_and_release_block(void*) <null>:1063616 (libclang_rt.tsan_osx_dynamic.dylib:x86_64h+0x6657b) #3 _dispatch_client_callout <null>:1063616 (libdispatch.dylib:x86_64+0x1f63) Previous read of size 8 at 0x0001015c51b0 by main thread: #0 main assign.swift:9 (assign:x86_64+0x1000017ef) Location is global 'a' at 0x0001015c51b0 (assign+0x0001000021b0) Thread T1 (tid=4890272, running) is a GCD worker thread