C#: Auto-parenthesize IsPattern with or/and combinators

[knutwannheden]

Motivation

MergeIfWithParentIf merges nested if-statements using CSharpTemplate.Expression($"{_left} && {_right}"). When the outer condition is an IsPattern with an or combinator, the merged result changes semantics because C# && interacts with the pattern combinator: x is A or B && y parses as x is (A or (B && y)) instead of the intended (x is A or B) && y. This also causes a downstream NullReferenceException in CSharpPrinter.VisitIsPattern.

Examples

Before (wrong semantics):

if (existsMode is FtpRemoteExists.Resume or FtpRemoteExists.AddToEnd && fileExists)

After (correct):

if ((existsMode is FtpRemoteExists.Resume or FtpRemoteExists.AddToEnd) && fileExists)

Summary

• CSharpPrecedences.GetPrecedence is now context-aware for IsPattern: when the pattern contains an or combinator, effective precedence is lowered to 1 (same as ||); and combinator lowers it to 2 (same as &&)
• CSharpParenthesizeVisitor.NeedsParenthesesInContext no longer wraps CsBinary Or/And pattern combinators inside IsPattern — these are legitimate patterns, not boolean expressions
• Added ParenthesizeDeep public entry point for recursive parenthesization
• Clarified CSharpTemplate.Apply comment: inner precedence is handled by ApplySubstitutions, MaybeParenthesize only checks the graft site

Test plan

• New unit test: IsPattern with or combinator inside Binary(&&) gets parenthesized via MaybeParenthesize
• New unit test: plain IsPattern (no combinator) inside Binary(&&) is NOT parenthesized
• New unit test: ParenthesizeDeep recursive walk parenthesizes inner IsPattern with or inside Binary(&&)
• New unit test: ParenthesizeDeep does NOT parenthesize simple IsPattern inside Binary(&&)
• All 59 existing precedence and parenthesization tests continue to pass