UnauthorizedAccessException on pdb trying to debug a unit test - visual-studio

Question

If I put a single breakpoint on this method, on the last line, and debug via "Debug selected tests" in the Test Explorer, I get an UnauthorizedAccessException on the pdb of the test assembly and the test aborts.
module Tests
open System
open Xunit
[<Fact>]
let fact () =
let rng = Random ()
let a = rng.Next() % 2 = 0
let b = rng.Next() % 2 = 0
a && b // <- breakpoint here
If I put a breakpoint anywhere else, I can debug fine. I can put a breakpoint on that line and debug if there's a breakpoint somewhere else in the method; the last breakpoint just appears empty (no generated code matches it). Note that the && on the last line seems important, if I remove it the problem goes away. For example this version doesn't exhibit the problem:
[<Fact>]
let fact () =
let rng = Random ()
let a = rng.Next() % 2 = 0
let b = rng.Next() % 2 = 0
let result = a && b
result // can put a breakpoint anywhere and debug fine
I could not repro outside a particular solution containing many other projects that I work on, even using all the exact same references and App.config. I spent already quite some time trying to nail down the issue and am looking for tips at this point. Here they are anyway:
App.config:
<?xml version="1.0" encoding="utf-8" ?>
<configuration>
<appSettings>
<add key="xunit.methodDisplay" value="method"/>
</appSettings>
<runtime>
<assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
<dependentAssembly>
<assemblyIdentity name="xunit.core" publicKeyToken="8d05b1bb7a6fdb6c" culture="neutral" />
<bindingRedirect oldVersion="0.0.0.0-2.1.0.3179" newVersion="2.1.0.3179" />
</dependentAssembly>
<dependentAssembly>
<assemblyIdentity name="FSharp.Core" publicKeyToken="b03f5f7f11d50a3a" culture="neutral" />
<bindingRedirect oldVersion="0.0.0.0-4.4.0.0" newVersion="4.4.0.0" />
</dependentAssembly>
</assemblyBinding>
</runtime>
</configuration>
packages.config:
<?xml version="1.0" encoding="utf-8"?>
<packages>
<package id="FSharp.Core" version="4.0.0.1" targetFramework="net46" />
<package id="xunit" version="2.1.0" targetFramework="net46" />
<package id="xunit.abstractions" version="2.0.0" targetFramework="net46" />
<package id="xunit.assert" version="2.1.0" targetFramework="net46" />
<package id="xunit.core" version="2.1.0" targetFramework="net46" />
<package id="xunit.extensibility.core" version="2.1.0" targetFramework="net46" />
<package id="xunit.extensibility.execution" version="2.1.0" targetFramework="net46" />
<package id="xunit.runner.visualstudio" version="2.1.0" targetFramework="net46" />
</packages>
Visual Studio Enterprise 2015 Update 3 (Version 14.0.25424.00)

Answer 1

It's a bit of guesswork, but the a && b is an expression rather than a statement, so the breakpoint doesn't get hit.
If the test runner has made a shadow copy of your assembly and PDB, then it might try to delete it after the test completes - but the debugger is still accessing it, leading to the exception.

Answer 2

Assuming you are on the same bitness (using 32bit xunit runner for a 32 bit project), I'm guessing the reason this is breaking is because unit tests should have signature: unit -> unit or in C# terms void.
If you change your first example to return unit, I was able to debug a && b with no issues. Now as to why your second example works even though it has the same return type, I have no idea. Maybe the bug is related to needing to assign a variable to be able do stuff with it in xunit?
[<Fact>]
let fact () =
let rng = Random ()
let a = rng.Next() % 2 = 0
let b = rng.Next() % 2 = 0
a && b // <- breakpoint here works
()