Ruby: Add CaseElseBranch AST node to distinguish else-branch from its body

[Copilot]

In Ruby's case expression, the else branch was directly represented as a StmtSequence, unlike WhenClause and InClause which both have a body distinct from the branch node itself. This inconsistency made it impossible to refer to the else-branch as a node separate from its contents.

Changes

New AST node: CaseElseBranch

• CaseExpr.getElseBranch() now returns a CaseElseBranch instead of StmtSequence
• CaseElseBranch.getBody() returns the StmtSequence body — mirrors WhenClause.getBody() / InClause.getBody()

case foo
when 1 then puts "one"
else puts "other"   # <- CaseElseBranch node; getBody() returns the StmtSequence
end

Implementation (synthesized node approach)

• Added CaseElseBranchKind / TCaseElseBranchSynth to the synthesis system — a wrapper node injected between the CaseExpr and the existing Else/StmtSequence real node, preserving location from the underlying tree-sitter Else node
• CaseWhenClause.getBranch and CaseMatch.getBranch now return the synthesized wrapper instead of the raw Else node for the else index
• TestPatternDesugar (expr in pattern) updated to wrap its synthesized ElseSynth in a CaseElseBranchSynth

CFG

• Added CaseElseBranchTree — transparent to control flow; delegates first/last to the body

Breaking change

• CaseExpr.getElseBranch() return type changed from StmtSequence to CaseElseBranch; callers needing the body should call .getBody()

[Copilot]

Pull request overview

This pull request updates the Ruby AST model for case expressions to represent the else branch as a distinct CaseElseBranch node (separate from its body), aligning it more closely with WhenClause / InClause structure and enabling callers to refer to the else-branch node itself.

Changes:

• Introduces CaseElseBranch as a new AST node and updates CaseExpr.getElseBranch() to return it (breaking change).
• Implements CaseElseBranch via synthesized-node wrapping in the AST synthesis system, with corresponding internal AST updates.
• Updates control-flow and dataflow handling plus library tests/expected outputs and adds a breaking change note.

Show a summary per file

File	Description
ruby/ql/test/library-tests/modules/modules.expected	Updates expected module-enclosing output for the new else-branch node.
ruby/ql/test/library-tests/modules/methods.expected	Updates expected method-enclosing output for the new else-branch node.
ruby/ql/test/library-tests/ast/control/CaseExpr.ql	Adjusts library-test query to reflect getElseBranch()’s new return type.
ruby/ql/test/library-tests/ast/AstDesugar.expected	Updates desugaring AST expectations to include CaseElseBranch and its body.
ruby/ql/test/library-tests/ast/Ast.expected	Updates AST expectations to show CaseElseBranch wrapping the else body.
ruby/ql/lib/codeql/ruby/dataflow/internal/DataFlowPrivate.qll	Updates case-branch last-eval logic to follow else branch via .getElseBranch().getBody().
ruby/ql/lib/codeql/ruby/controlflow/internal/ControlFlowGraphImpl.qll	Adds a transparent CFG tree wrapper for CaseElseBranch delegating to its body.
ruby/ql/lib/codeql/ruby/ast/internal/Synthesis.qll	Adds synth kind + synthesis wiring for CaseElseBranch and updates pattern desugar synthesis.
ruby/ql/lib/codeql/ruby/ast/internal/Control.qll	Wraps case/in else branches with the synthesized CaseElseBranch node; adds implementation class.
ruby/ql/lib/codeql/ruby/ast/internal/AST.qll	Registers TCaseElseBranchSynth in cached AST node kinds/classes.
ruby/ql/lib/codeql/ruby/ast/Control.qll	Exposes public CaseElseBranch API and updates CaseExpr branch/else docs and return type.
ruby/ql/lib/change-notes/2026-06-15-case-else-branch.md	Adds breaking-change release note for the getElseBranch() return type change.

Copilot's findings

• Files reviewed: 12/12 changed files
• Comments generated: 1