F# 6, nullable and implicit upcasts

[NinoFloris]

Found while working on Giraffe, trying to set HttpContext.Response.ContentLength (which expects a Nullable<int64>) the F# compiler gladly accepts code like this:

let x: int = 0
ctx.Response.ContentLength <- x

Which then fails at runtime with System.InvalidCastException: Unable to cast object of type 'System.Int32' to type 'System.Int64'.

Difference at the IL level

Runtime exception:

      IL_001d: ldloc.0      // r
      IL_001e: ldarg.0      // this
      IL_001f: ldfld        unsigned int8[] App.Main/main@151::hw
      IL_0024: ldlen
      IL_0025: conv.i4
      IL_0026: conv.i8
      IL_0027: box          [System.Runtime]System.Int32
      IL_002c: unbox.any    [System.Runtime]System.Int64
      IL_0031: newobj       instance void valuetype [System.Runtime]System.Nullable`1<int64>::.ctor(!0/*int64*/)
      IL_0036: callvirt     instance void [Microsoft.AspNetCore.Http.Abstractions]Microsoft.AspNetCore.Http.HttpResponse::set_ContentLength(valuetype [System.Runtime]System.Nullable`1<int64>)

int64 cast inserted manually:

      IL_001d: ldloc.0      // r
      IL_001e: ldarg.0      // this
      IL_001f: ldfld        unsigned int8[] App.Main/main@151::hw
      IL_0024: ldlen
      IL_0025: conv.i4
      IL_0026: conv.i8
      IL_0027: newobj       instance void valuetype [System.Runtime]System.Nullable`1<int64>::.ctor(!0/*int64*/)
      IL_002c: callvirt     instance void [Microsoft.AspNetCore.Http.Abstractions]Microsoft.AspNetCore.Http.HttpResponse::set_ContentLength(valuetype [System.Runtime]System.Nullable`1<int64>)

Somehow the box + unbox is breaking things due to us boxing it as int32 again

[NinoFloris]

@dsyme this is another F# 6 codegen issue.

[dsyme]

@NinoFloris Thank you, we will get this fixed.

[dsyme]

Fix is at #12423

As workaround to avoid this you can enable

	  <WarnOn>3389</WarnOn>
	  <WarningsAsErrors>3389</WarningsAsErrors>

This will give an error every time an implicit int32 --> int64 conversion is used.