System.Range not included in NetFramework 4.6.2 projects #96

Description (optional)

I have several NetFramework 4.6.2 projects in a solution and when compiling them, I noticed build issues when using the new range syntax for strings. I replaced the usages of range with SubString calls and compiled the solution then opened the compiled assembly and noticed that the System.Range has not been included.

Maybe I'm doing something wrong? but, I wasn't able to find anything about this in the docs.

Reproduction Steps

1. Build a .Net Framework 4.6.2 Library project.
2. Use range syntax in a method (e.g. "string"[..2]).
3. Build the project.
4. Build will fails, saying that System.Range does not exist.
5. Replace the range with SubString.
6. Build again.
7. Use a decompiler to open the assembly.
8. You will notice System.Range is not included.

Expected Behavior

The assembly should contain System.Range and the range syntax should work.

Actual Behavior

If range syntax is used the build will fail. If range syntax is not used, build is successful, but the System.Range won't be included.

System info

This section should contain useful info such as:

• PolySharp NuGet version (or branch/commit, if building from source)
• Operating system version
• Visual Studio version, or .NET SDK/runtime version
  • VS Version: 17.8.6
  • Polysharp: 1.14.1
  • .NET: 4.6.2
  • LangVersion: 12.0
  • Project is using the new PackageReference syntax.

Additional context (optional)

I did not notice any messages from Polysharp in the build log, even when building with detailed logging.