Code using INumber<T> and preview features does not compile with .NET 6.0.101 SDK

[martincostello]

Describe the bug

An application using INumber<T> with EnablePreviewFeatures set to true does not compile using the .NET 6.0.101 SDK.

The same code compiles with the .NET 6.0.100 SDK.

To Reproduce

Clone the update-dotnet-sdk-6.0.101 branch of this repository and run dotnet build.

Errors

Error: D:\a\adventofcode\adventofcode\src\AdventOfCode\Maths.cs(54,19): error CS0246: The type or namespace name 'INumber<>' could not be found (are you missing a using directive or an assembly reference?) [D:\a\adventofcode\adventofcode\src\AdventOfCode\AdventOfCode.csproj]
Error: D:\a\adventofcode\adventofcode\src\AdventOfCode\Maths.cs(79,24): error CS0246: The type or namespace name 'INumber<>' could not be found (are you missing a using directive or an assembly reference?) [D:\a\adventofcode\adventofcode\src\AdventOfCode\AdventOfCode.csproj]
Error: D:\a\adventofcode\adventofcode\src\AdventOfCode\Maths.cs(80,24): error CS0246: The type or namespace name 'INumber<>' could not be found (are you missing a using directive or an assembly reference?) [D:\a\adventofcode\adventofcode\src\AdventOfCode\AdventOfCode.csproj]
Error: D:\a\adventofcode\adventofcode\src\AdventOfCode\Maths.cs(80,41): error CS0246: The type or namespace name 'IComparisonOperators<,>' could not be found (are you missing a using directive or an assembly reference?) [D:\a\adventofcode\adventofcode\src\AdventOfCode\AdventOfCode.csproj]
Error: D:\a\adventofcode\adventofcode\src\AdventOfCode\Maths.cs(138,19): error CS0246: The type or namespace name 'INumber<>' could not be found (are you missing a using directive or an assembly reference?) [D:\a\adventofcode\adventofcode\src\AdventOfCode\AdventOfCode.csproj]
Error: D:\a\adventofcode\adventofcode\src\AdventOfCode\Maths.cs(195,19): error CS0246: The type or namespace name 'INumber<>' could not be found (are you missing a using directive or an assembly reference?) [D:\a\adventofcode\adventofcode\src\AdventOfCode\AdventOfCode.csproj]
Error: D:\a\adventofcode\adventofcode\src\AdventOfCode\Maths.cs(217,19): error CS0246: The type or namespace name 'INumber<>' could not be found (are you missing a using directive or an assembly reference?) [D:\a\adventofcode\adventofcode\src\AdventOfCode\AdventOfCode.csproj]
Error: D:\a\adventofcode\adventofcode\src\AdventOfCode\Puzzle.cs(85,19): error CS0246: The type or namespace name 'INumber<>' could not be found (are you missing a using directive or an assembly reference?) [D:\a\adventofcode\adventofcode\src\AdventOfCode\AdventOfCode.csproj]
Error: D:\a\adventofcode\adventofcode\src\AdventOfCode\Puzzle.cs(97,19): error CS0246: The type or namespace name 'INumber<>' could not be found (are you missing a using directive or an assembly reference?) [D:\a\adventofcode\adventofcode\src\AdventOfCode\AdventOfCode.csproj]
Error: D:\a\adventofcode\adventofcode\src\AdventOfCode\Puzzle.cs(113,19): error CS0246: The type or namespace name 'INumber<>' could not be found (are you missing a using directive or an assembly reference?) [D:\a\adventofcode\adventofcode\src\AdventOfCode\AdventOfCode.csproj]
Error: D:\a\adventofcode\adventofcode\src\AdventOfCode\Puzzle.cs(130,19): error CS0246: The type or namespace name 'INumber<>' could not be found (are you missing a using directive or an assembly reference?) [D:\a\adventofcode\adventofcode\src\AdventOfCode\AdventOfCode.csproj]
Error: D:\a\adventofcode\adventofcode\src\AdventOfCode\Puzzle.cs(197,19): error CS0246: The type or namespace name 'INumber<>' could not be found (are you missing a using directive or an assembly reference?) [D:\a\adventofcode\adventofcode\src\AdventOfCode\AdventOfCode.csproj]
Error: D:\a\adventofcode\adventofcode\src\AdventOfCode\DictionaryExtensions.cs(20,24): error CS0246: The type or namespace name 'INumber<>' could not be found (are you missing a using directive or an assembly reference?) [D:\a\adventofcode\adventofcode\src\AdventOfCode\AdventOfCode.csproj]
Error: D:\a\adventofcode\adventofcode\src\AdventOfCode\DictionaryExtensions.cs(33,24): error CS0246: The type or namespace name 'INumber<>' could not be found (are you missing a using directive or an assembly reference?) [D:\a\adventofcode\adventofcode\src\AdventOfCode\AdventOfCode.csproj]
Error: D:\a\adventofcode\adventofcode\src\AdventOfCode\StringExtensions.cs(42,19): error CS0246: The type or namespace name 'INumber<>' could not be found (are you missing a using directive or an assembly reference?) [D:\a\adventofcode\adventofcode\src\AdventOfCode\AdventOfCode.csproj]

Further technical details

> dotnet --info
.NET SDK (reflecting any global.json):
 Version:   6.0.101
 Commit:    ef49f6213a

Runtime Environment:
 OS Name:     Windows
 OS Version:  10.0.19043
 OS Platform: Windows
 RID:         win10-x64
 Base Path:   C:\Program Files\dotnet\sdk\6.0.101\

Host (useful for support):
  Version: 6.0.1
  Commit:  3a25a7f1cc

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[martincostello]

Tracked by dotnet/runtime#62840.