using System;
module Program
{
  Main() : void
  {
      mutable arg;
      Test(out arg);
  }
  
  Test(arg : out Struct1) : void
  {
    arg.Field1 = if (true) "1"; else "2";
  }
  
  public struct Struct1
  {
    public mutable Field1 : string;
  }
}

Application: ConsoleApplication1.exe
Framework Version: v4.0.30319
Description: The process was terminated due to an internal error in the .NET Runtime at IP 6D683A86 (6D680000) with exit code 80131506.

Faulting application name: ConsoleApplication1.exe, version: 0.0.0.0, time stamp: 0x4e3116ba
Faulting module name: clr.dll, version: 4.0.30319.1, time stamp: 0x4ba1d9ef
Exception code: 0xc0000005
Fault offset: 0x00003a86
Faulting process id: 0xd4
Faulting application start time: 0x01cc4d0009c1d15c
Faulting application path: Z:\Shared Folders\ConsoleApplication1.exe
Faulting module path: C:\Windows\Microsoft.NET\Framework\v4.0.30319\clr.dll

using System;
module Program
{
  Main() : void
  {
      mutable arg;
      Test(out arg);
  }
  
  Test(arg : out Struct1) : void
  {
    arg.Field1 = if (true) 1; else 2; // здесь NRE на win64
  }
  
  public struct Struct1
  {
    public mutable Field1 : int;
  }
}

using System;
M>module Program
M>{
M>  Main() : void
M>  {
M>      mutable arg;
M>      Test(out arg);
M>  }
  
M>  Test(arg : out Struct1) : void
M>  {
M>    arg.Field1 = if (true) "1"; else "2";
M>  }
  
M>  public struct Struct1
M>  {
M>    public mutable Field1 : string;
M>  }
M>}
M>

E:\MyProjects\Tests\ConsoleApplication355\ConsoleApplication355\bin\Debug>"E:\Program Files (x86)\Microsoft SDKs\Windows\v7.0A\Bin\NETFX 4.0 Tools\PEVerify.
exe" ConsoleApplication355.exe

Microsoft (R) .NET Framework PE Verifier. Version 4.0.30319.1
Copyright (c) Microsoft Corporation. All rights reserved.

[IL]: Error: [E:\MyProjects\Tests\ConsoleApplication355\ConsoleApplication355\bin\Debug\ConsoleApplication355.exe : Program::Test][offset 0x00000003][found
address of value 'Program+Struct1'][expected value 'Program+Struct1'] Unexpected type on the stack.
[IL]: Error: [E:\MyProjects\Tests\ConsoleApplication355\ConsoleApplication355\bin\Debug\ConsoleApplication355.exe : Program::Test][offset 0x00000021][found
value 'Program+Struct1'][expected readonly address of value 'Program+Struct1'] Unexpected type on the stack.
2 Error(s) Verifying ConsoleApplication355.exe

if (condition)
  arg.Field1 = "1";
else
  arg.Field2 = "2";

arg.Field1 = if (condition) "1"; else "2";

using System;
module Program
{
  Main() : void
  {
    def a = Struct1();
    a.Field1 = if (true) 1; else 2;
  }
  
  public struct Struct1
  {
    public mutable Field1 : int;
  }
}