C#7.2 produces not verifiable il code when direct accessing field of readonly struct field

[Ralf1108]

Version Used:
Visual Studio 2017
LanguageVersion: C# 7.2

Steps to Reproduce:

1. Compile program in DEBUG with 'x1' or 'x2' uncommented:

namespace PeVerifyBug
{
    class Program
    {
        static void Main(string[] args)
        {
        }
    }

    public class TestClass
    {
        private readonly TestStruct _readonlyValue = new TestStruct(42);

        public TestStruct Value
        {
            get
            {
                //var x1 = _readonlyValue.X;
                //var x2 = _readonlyValue.XX;
                return default(TestStruct);
            }
        }
    }

    public struct TestStruct
    {
        public readonly float X;

        public TestStruct(float x)
        {
            X = x;
        }

        public float XX => X;
    }
}

2. Run PE-Verify on the result assembly
   I used the one from "C:\Program Files (x86)\Microsoft SDKs\Windows\v10.0A\bin\NETFX 4.7.1 Tools"
   -> Microsoft (R) .NET Framework PE Verifier. Version 4.0.30319.0

Expected Behavior:
pe verifier should not complain.

Actual Behavior:

• if 'x1' was uncommented, pe verifier complains about:
  [IL]: Error: [C:\Users\xxxx\source\repos\PeVerifyBug\PeVerifyBug\bin\Debug\PeVerifyBug.exe : PeVerifyBug.TestClass::get_Value][offset 0x00000002] Cannot change initonly field outside its .ctor.
• if 'x2' was uncommented, pe verifier doesn't complain.

Analyzing the il code

• 'x1'-version:

IL_0000: nop
IL_0001: ldarg.0
IL_0002: ldflda valuetype PeVerifyBug.TestStruct PeVerifyBug.TestClass::_readonlyValue
IL_0007: ldfld float32 PeVerifyBug.TestStruct::X
IL_000c: stloc.0
IL_000d: ldloca.s 1
IL_000f: initobj PeVerifyBug.TestStruct
IL_0015: ldloc.1
IL_0016: stloc.2
IL_0017: br.s IL_0019

IL_0019: ldloc.2
IL_001a: ret

• 'x2'-version:

IL_0000: nop
IL_0001: ldarg.0
IL_0002: ldfld valuetype PeVerifyBug.TestStruct PeVerifyBug.TestClass::_readonlyValue
IL_0007: stloc.1
IL_0008: ldloca.s 1
IL_000a: call instance float32 PeVerifyBug.TestStruct::get_XX()
IL_000f: stloc.0
IL_0010: ldloca.s 1
IL_0012: initobj PeVerifyBug.TestStruct
IL_0018: ldloc.1
IL_0019: stloc.2
IL_001a: br.s IL_001c

IL_001c: ldloc.2
IL_001d: ret

Additional information
When using LanguageVersion: C# 7.1 both cases work
so I think this is a compiler bug and not a problem of PE verify.

[jcouv]

Relates to #21269 and #22772
The C# compiler sometimes emit unverifiable code (by current PEVerify rules). This can happen when using new features, or when re-compiling existing code with LangVersion >= 7.2. In the latter case, a feature flag can be set in the project to restore the non-optimized behavior (if you prefer that).

[Ralf1108]

solved it with introducing property getter instead of directly access the fields. Thanks for clarification!