Add missing string.Join polyfills

[Copilot]

Implements missing string.Join polyfills as tracked in the API reference.

Changes

PolyShim/NetCore10/String.cs

Adds two overloads introduced in .NET Framework 4.0 (missing on net35):

• Join(string? separator, params object?[] values)
• Join<T>(string? separator, IEnumerable<T> values)

PolyShim/NetCore20/String.cs

Adds four char-separator overloads introduced in .NET Core 2.0 (missing on all .NET Framework, older .NET Core, and .NET Standard < 2.1), consolidated into the existing extension(string str) block:

• Join(char separator, params object?[] values)
• Join(char separator, params string?[] values)
• Join<T>(char separator, IEnumerable<T> values)
• Join(char separator, string?[] value, int startIndex, int count)

Note: Join(string? separator, string?[] value, int startIndex, int count) has been available since .NET Framework 1.1 and does not require a polyfill.

Tests

• Extended PolyShim.Tests/NetCore20/StringTests.cs — tests for the char-separator Join overloads

Security Summary

No security vulnerabilities introduced or discovered.

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[Tyrrrz]

@copilot

[Tyrrrz]

@copilot tests failing

[Copilot]

@copilot tests failing

The CI failure was in NetCore10/StringTests.cs running on net462. The string.Join(string, object[]) polyfill is only active on net35, so on net462 the native .NET Framework implementation was invoked — which has a known quirk of returning "" when the first array element is null. This was fixed in 22a38e0 by removing the test file (as requested in the review).

[Copilot]

Pull request overview

Adds missing string.Join overload polyfills to PolyShim to align with the .NET API surface across older TFMs, and updates the signature inventory + tests accordingly.

Changes:

• Added string.Join(string?, params object?[]) and string.Join<T>(string?, IEnumerable<T>) polyfills for older .NET Framework targets (net35).
• Added string.Join char-separator overload polyfills (object[], string[], IEnumerable<T>, and indexed slice) for frameworks where they’re missing.
• Updated Signatures.md counts/entries and added tests for the new char-separator overloads.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
PolyShim/Signatures.md	Updates signature totals and adds entries for the newly polyfilled string.Join overloads.
PolyShim/NetCore10/String.cs	Adds .NET 4.0-era Join overload polyfills for older .NET Framework.
PolyShim/NetCore20/String.cs	Adds .NET Core 2.0-era char-separator Join overload polyfills.
PolyShim.Tests/NetCore20/StringTests.cs	Adds test coverage for the new char-separator Join overloads.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.48%. Comparing base (b64504b) to head (22a38e0).
⚠️ Report is 1 commits behind head on prime.

Additional details and impacted files

@@            Coverage Diff             @@
##            prime      #90      +/-   ##
==========================================
+ Coverage   85.40%   85.48%   +0.08%     
==========================================
  Files         220      220              
  Lines        3603     3630      +27     
  Branches      314      315       +1     
==========================================
+ Hits         3077     3103      +26     
- Misses        454      456       +2     
+ Partials       72       71       -1     

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