Merge pull request #21991 from github/copilot/change-ast-for-else-branches

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

Author: aschackmull

ruby/ql/lib/change-notes/2026-06-15-case-else-branch.md

@@ -0,0 +1,4 @@
+---
+category: breaking
+---
+* The `else` branch of a `case` expression is no longer represented as a `StmtSequence` directly. Instead, a new `CaseElseBranch` AST node wraps the body (a `StmtSequence`). `CaseExpr.getElseBranch()` now returns a `CaseElseBranch`, and the body of the else branch can be accessed via `CaseElseBranch.getBody()`.

ruby/ql/lib/codeql/ruby/ast/Control.qll

@@ -377,18 +377,18 @@ class CaseExpr extends ControlExpr instanceof CaseExprImpl {
 
   /**
    * Gets the `n`th branch of this case expression, either a `WhenClause`, an
-   * `InClause`, or a `StmtSequence`.
+   * `InClause`, or a `CaseElseBranch`.
    */
   final AstNode getBranch(int n) { result = super.getBranch(n) }
 
   /**
    * Gets a branch of this case expression, either a `WhenClause`, an
-   * `InClause`, or a `StmtSequence`.
+   * `InClause`, or a `CaseElseBranch`.
    */
   final AstNode getABranch() { result = this.getBranch(_) }
 
   /** Gets the `else` branch of this case expression, if any. */
-  final StmtSequence getElseBranch() { result = this.getABranch() }
+  final CaseElseBranch getElseBranch() { result = this.getABranch() }
 
   /**
    * Gets the number of branches of this case expression.
@@ -533,6 +533,30 @@ class InClause extends AstNode instanceof InClauseImpl {
   }
 }
 
+/**
+ * An `else` branch of a `case` expression.
+ * ```rb
+ * case foo
+ * when 1 then puts "one"
+ * else puts "other"
+ * end
+ * ```
+ */
+class CaseElseBranch extends AstNode instanceof CaseElseBranchImpl {
+  final override string getAPrimaryQlClass() { result = "CaseElseBranch" }
+
+  /** Gets the body of this else branch. */
+  final StmtSequence getBody() { result = super.getBody() }
+
+  final override string toString() { result = "else ..." }
+
+  final override AstNode getAChild(string pred) {
+    result = AstNode.super.getAChild(pred)
+    or
+    pred = "getBody" and result = this.getBody()
+  }
+}
+
 /**
  * A one-line pattern match using the `=>` operator. For example:
  * ```rb

ruby/ql/lib/codeql/ruby/ast/internal/AST.qll

@@ -113,6 +113,9 @@ private module Cached {
     TBraceBlockSynth(Ast::AstNode parent, int i) { mkSynthChild(BraceBlockKind(), parent, i) } or
     TBraceBlockReal(Ruby::Block g) { not g.getParent() instanceof Ruby::Lambda } or
     TBreakStmt(Ruby::Break g) or
+    TCaseElseBranchSynth(Ast::AstNode parent, int i) {
+      mkSynthChild(CaseElseBranchKind(), parent, i)
+    } or
     TCaseEqExpr(Ruby::Binary g) { g instanceof @ruby_binary_equalequalequal } or
     TCaseExpr(Ruby::Case g) or
     TCaseMatchReal(Ruby::CaseMatch g) or
@@ -400,14 +403,15 @@ private module Cached {
   class TAstNodeSynth =
     TAddExprSynth or TAssignExprSynth or TBitwiseAndExprSynth or TBitwiseOrExprSynth or
         TBitwiseXorExprSynth or TBraceBlockSynth or TBodyStmtSynth or TBooleanLiteralSynth or
-        TCaseMatchSynth or TClassVariableAccessSynth or TConstantReadAccessSynth or
-        TConstantWriteAccessSynth or TDivExprSynth or TElseSynth or TExponentExprSynth or
-        TGlobalVariableAccessSynth or TIfSynth or TInClauseSynth or TInstanceVariableAccessSynth or
-        TIntegerLiteralSynth or TLShiftExprSynth or TLocalVariableAccessSynth or
-        TLogicalAndExprSynth or TLogicalOrExprSynth or TMethodCallSynth or TModuloExprSynth or
-        TMulExprSynth or TNilLiteralSynth or TRShiftExprSynth or TRangeLiteralSynth or TSelfSynth or
-        TSimpleParameterSynth or TSplatExprSynth or THashSplatExprSynth or TStmtSequenceSynth or
-        TSubExprSynth or TPairSynth or TSimpleSymbolLiteralSynth;
+        TCaseElseBranchSynth or TCaseMatchSynth or TClassVariableAccessSynth or
+        TConstantReadAccessSynth or TConstantWriteAccessSynth or TDivExprSynth or TElseSynth or
+        TExponentExprSynth or TGlobalVariableAccessSynth or TIfSynth or TInClauseSynth or
+        TInstanceVariableAccessSynth or TIntegerLiteralSynth or TLShiftExprSynth or
+        TLocalVariableAccessSynth or TLogicalAndExprSynth or TLogicalOrExprSynth or
+        TMethodCallSynth or TModuloExprSynth or TMulExprSynth or TNilLiteralSynth or
+        TRShiftExprSynth or TRangeLiteralSynth or TSelfSynth or TSimpleParameterSynth or
+        TSplatExprSynth or THashSplatExprSynth or TStmtSequenceSynth or TSubExprSynth or
+        TPairSynth or TSimpleSymbolLiteralSynth;
 
   /**
    * Gets the underlying TreeSitter entity for a given AST node. This does not
@@ -598,6 +602,8 @@ private module Cached {
     or
     result = TBraceBlockSynth(parent, i)
     or
+    result = TCaseElseBranchSynth(parent, i)
+    or
     result = TCaseMatchSynth(parent, i)
     or
     result = TClassVariableAccessSynth(parent, i, _)
@@ -718,6 +724,8 @@ TAstNodeReal fromGenerated(Ruby::AstNode n) { n = toGenerated(result) }
 
 class TCall = TMethodCall or TYieldCall;
 
+class TCaseElseBranch = TCaseElseBranchSynth;
+
 class TCaseMatch = TCaseMatchReal or TCaseMatchSynth;
 
 class TCase = TCaseExpr or TCaseMatch;

ruby/ql/lib/codeql/ruby/ast/internal/Control.qll

@@ -19,8 +19,11 @@ class CaseWhenClause extends CaseExprImpl, TCaseExpr {
   final override Expr getValue() { toGenerated(result) = g.getValue() }
 
   final override AstNode getBranch(int n) {
-    toGenerated(result) = g.getChild(n) or
-    toGenerated(result) = g.getChild(n)
+    // When branches map directly to WhenClause nodes
+    toGenerated(result) = g.getChild(n) and not g.getChild(n) instanceof Ruby::Else
+    or
+    // The else branch is wrapped in a synthesized CaseElseBranch node
+    g.getChild(n) instanceof Ruby::Else and result = getSynthChild(this, n)
   }
 }
 
@@ -34,7 +37,8 @@ class CaseMatch extends CaseExprImpl, TCaseMatchReal {
   final override AstNode getBranch(int n) {
     toGenerated(result) = g.getClauses(n)
     or
-    n = count(g.getClauses(_)) and toGenerated(result) = g.getElse()
+    // The else branch is wrapped in a synthesized CaseElseBranch node
+    n = count(g.getClauses(_)) and exists(g.getElse()) and result = getSynthChild(this, n)
   }
 }
 
@@ -87,3 +91,9 @@ class InClauseSynth extends InClauseImpl, TInClauseSynth {
 
   final override predicate hasUnlessCondition() { none() }
 }
+
+class CaseElseBranchImpl extends AstNode, TCaseElseBranch {
+  CaseElseBranchImpl() { this = TCaseElseBranchSynth(_, _) }
+
+  final StmtSequence getBody() { synthChild(this, 0, result) }
+}

ruby/ql/lib/codeql/ruby/ast/internal/Synthesis.qll

@@ -22,6 +22,7 @@ newtype TSynthKind =
   BodyStmtKind() or
   BooleanLiteralKind(boolean value) { value = true or value = false } or
   BraceBlockKind() or
+  CaseElseBranchKind() or
   CaseMatchKind() or
   ClassVariableAccessKind(ClassVariable v) or
   DefinedExprKind() or
@@ -80,6 +81,8 @@ class SynthKind extends TSynthKind {
     or
     this = BraceBlockKind() and result = "BraceBlockKind"
     or
+    this = CaseElseBranchKind() and result = "CaseElseBranchKind"
+    or
     this = CaseMatchKind() and result = "CaseMatchKind"
     or
     this = ClassVariableAccessKind(_) and result = "ClassVariableAccessKind"
@@ -1840,7 +1843,7 @@ private module TestPatternDesugar {
           or
           child = SynthChild(InClauseKind()) and i = 1
           or
-          child = SynthChild(ElseKind()) and i = 2
+          child = SynthChild(CaseElseBranchKind()) and i = 2
         )
         or
         parent = TInClauseSynth(case, 1) and
@@ -1851,7 +1854,11 @@ private module TestPatternDesugar {
           child = SynthChild(BooleanLiteralKind(true)) and i = 1
         )
         or
-        parent = TElseSynth(case, 2) and
+        parent = TCaseElseBranchSynth(case, 2) and
+        child = SynthChild(ElseKind()) and
+        i = 0
+        or
+        parent = TElseSynth(TCaseElseBranchSynth(case, 2), 0) and
         child = SynthChild(BooleanLiteralKind(false)) and
         i = 0
       )
@@ -1994,3 +2001,61 @@ private module CallableBodySynthesis {
     }
   }
 }
+
+private module CaseElseBranchSynthesis {
+  pragma[nomagic]
+  private predicate caseElseBranchSynthesis(AstNode parent, int i, Child child) {
+    // Wrap the else branch of a real `case`/`when` expression
+    exists(Ruby::Case g, Ruby::Else elseNode, int elseIndex |
+      elseNode = g.getChild(elseIndex) and
+      (
+        // Create the CaseElseBranch wrapper node at the else index
+        parent = TCaseExpr(g) and
+        child = SynthChild(CaseElseBranchKind()) and
+        i = elseIndex
+        or
+        // The body of the CaseElseBranch is the Else node
+        parent = TCaseElseBranchSynth(TCaseExpr(g), elseIndex) and
+        child = RealChildRef(TElseReal(elseNode)) and
+        i = 0
+      )
+    )
+    or
+    // Wrap the else branch of a real `case`/`in` expression
+    exists(Ruby::CaseMatch g, Ruby::Else elseNode, int elseIndex |
+      elseNode = g.getElse() and
+      elseIndex = count(g.getClauses(_)) and
+      (
+        // Create the CaseElseBranch wrapper node at the else index
+        parent = TCaseMatchReal(g) and
+        child = SynthChild(CaseElseBranchKind()) and
+        i = elseIndex
+        or
+        // The body of the CaseElseBranch is the Else node
+        parent = TCaseElseBranchSynth(TCaseMatchReal(g), elseIndex) and
+        child = RealChildRef(TElseReal(elseNode)) and
+        i = 0
+      )
+    )
+  }
+
+  private class CaseElseBranchSynthesisImpl extends Synthesis {
+    final override predicate child(AstNode parent, int i, Child child) {
+      caseElseBranchSynthesis(parent, i, child)
+    }
+
+    final override predicate location(AstNode n, Location l) {
+      // Give the CaseElseBranch the location of the underlying Else node
+      exists(Ruby::Case g, int elseIndex |
+        n = TCaseElseBranchSynth(TCaseExpr(g), elseIndex) and
+        l = g.getChild(elseIndex).getLocation()
+      )
+      or
+      exists(Ruby::CaseMatch g, int elseIndex |
+        elseIndex = count(g.getClauses(_)) and
+        n = TCaseElseBranchSynth(TCaseMatchReal(g), elseIndex) and
+        l = g.getElse().getLocation()
+      )
+    }
+  }
+}

ruby/ql/lib/codeql/ruby/controlflow/internal/ControlFlowGraphImpl.qll

@@ -498,6 +498,16 @@ module Trees {
     }
   }
 
+  private class CaseElseBranchTree extends ControlFlowTree instanceof CaseElseBranch {
+    final override predicate propagatesAbnormal(AstNode child) { child = super.getBody() }
+
+    final override predicate first(AstNode first) { first(super.getBody(), first) }
+
+    final override predicate last(AstNode last, Completion c) { last(super.getBody(), last, c) }
+
+    final override predicate succ(AstNode pred, AstNode succ, Completion c) { none() }
+  }
+
   private class PatternVariableAccessTree extends LocalVariableAccessTree instanceof LocalVariableWriteAccess,
     CasePattern
   {

ruby/ql/lib/codeql/ruby/dataflow/internal/DataFlowPrivate.qll

@@ -68,9 +68,9 @@ private CfgNodes::ExprCfgNode getALastEvalNode(CfgNodes::ExprCfgNode n) {
     result = branch.(CfgNodes::ExprNodes::InClauseCfgNode).getBody()
     or
     result = branch.(CfgNodes::ExprNodes::WhenClauseCfgNode).getBody()
-    or
-    result = branch
   )
+  or
+  result.getAstNode() = n.(CfgNodes::ExprNodes::CaseExprCfgNode).getExpr().getElseBranch().getBody()
 }
 
 /**

ruby/ql/test/library-tests/ast/Ast.expected

@@ -815,8 +815,9 @@ control/cases.rb:
 #   11|       getPattern: [LocalVariableAccess] d
 #   11|       getBody: [StmtSequence] then ...
 #   12|         getStmt: [IntegerLiteral] 200
-#   13|     getBranch/getElseBranch: [StmtSequence] else ...
-#   14|       getStmt: [IntegerLiteral] 300
+#   13|     getBranch/getElseBranch: [CaseElseBranch] else ...
+#   13|       getBody: [StmtSequence] else ...
+#   14|         getStmt: [IntegerLiteral] 300
 #   18|   getStmt: [CaseExpr] case ...
 #   19|     getBranch: [WhenClause] when ...
 #   19|       getPattern: [GTExpr] ... > ...
@@ -843,8 +844,9 @@ control/cases.rb:
 #   27|       getPattern: [IntegerLiteral] 5
 #   27|       getBody: [StmtSequence] then ...
 #   27|         getStmt: [BooleanLiteral] true
-#   28|     getBranch/getElseBranch: [StmtSequence] else ...
-#   28|       getStmt: [BooleanLiteral] false
+#   28|     getBranch/getElseBranch: [CaseElseBranch] else ...
+#   28|       getBody: [StmtSequence] else ...
+#   28|         getStmt: [BooleanLiteral] false
 #   31|   getStmt: [CaseExpr] case ...
 #   31|     getValue: [MethodCall] call to expr
 #   31|       getReceiver: [SelfVariableAccess] self
@@ -862,8 +864,9 @@ control/cases.rb:
 #   34|         getAnOperand/getArgument/getGreaterOperand/getRightOperand: [IntegerLiteral] 0
 #   34|       getBody: [StmtSequence] then ...
 #   35|         getStmt: [BooleanLiteral] true
-#   36|     getBranch/getElseBranch: [StmtSequence] else ...
-#   36|       getStmt: [BooleanLiteral] false
+#   36|     getBranch/getElseBranch: [CaseElseBranch] else ...
+#   36|       getBody: [StmtSequence] else ...
+#   36|         getStmt: [BooleanLiteral] false
 #   39|   getStmt: [CaseExpr] case ...
 #   39|     getValue: [MethodCall] call to expr
 #   39|       getReceiver: [SelfVariableAccess] self

ruby/ql/test/library-tests/ast/AstDesugar.expected

@@ -389,8 +389,9 @@ control/cases.rb:
 #  160|         getPrefixElement: [IntegerLiteral] 1
 #  160|         getPrefixElement: [IntegerLiteral] 2
 #  160|       getBody: [BooleanLiteral] true
-#  160|     getBranch/getElseBranch: [StmtSequence] else ...
-#  160|       getStmt: [BooleanLiteral] false
+#  160|     getBranch/getElseBranch: [CaseElseBranch] else ...
+#  160|       getBody: [StmtSequence] else ...
+#  160|         getStmt: [BooleanLiteral] false
 #  162| [MatchPattern] ... => ...
 #  162|   getDesugared: [CaseExpr] case ...
 #  162|     getValue: [MethodCall] call to expr

ruby/ql/test/library-tests/ast/control/CaseExpr.ql

@@ -4,7 +4,7 @@ query predicate caseValues(CaseExpr c, Expr value) { value = c.getValue() }
 
 query predicate caseNoValues(CaseExpr c) { not exists(c.getValue()) }
 
-query predicate caseElseBranches(CaseExpr c, StmtSequence elseBranch) {
+query predicate caseElseBranches(CaseExpr c, CaseElseBranch elseBranch) {
   elseBranch = c.getElseBranch()
 }