[CSharp] CI build of Antlr CSharp runtime gives warnings

I don’t recall seeing this before, but under the “Files changed” tab for a PR, we now see warnings for legitimate build issues of the C# code.

Check warning on line 25 in runtime/CSharp/src/Tree/Xpath/XPathLexer.cs

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/ build (csharp)
runtime/CSharp/src/Tree/Xpath/XPathLexer.cs#L25
The using directive for 'System' appeared previously in this namespace

See https://github.com/antlr/antlr4/pull/3547/files

It’s caused because the grammar file XPathLexer.g4 includes an @header { using System;}. The @header is not needed. But, Char.IsUpper() should be fully qualified in order to eliminate the assumption that System is included. Honestly, the grammar shouldn’t have target-specific actions in the code, and should be in “target-agnostic format”. But, that’s another story.

But, it gets better…

The XPathLexer.cs is generated by the Antlr4 tool. It says that it was generated using 4.9.3, but it was not! I don’t know what tool was used, but it’s not the official 4.9.3 Antlr tool! The tables produced are different.

[KvanTTT]

Also, it is not covered by tests except for Java.

[KvanTTT]

I support the idea of dropping XPath from ANTLR core at all since it’s outdated and is not supported by some runtimes.