Add Parenthesized flag to Lambda AST node to fix multi-param lambda merging

When parsing multi-param lambdas like `acc,x -> body` where parameters are
separated by commas without parentheses, the parser was incorrectly merging
preceding identifiers with explicitly parenthesized lambdas like `(x -> y)`.

This fix:
- Adds `Parenthesized bool` field to ast.Lambda
- Sets this flag when a lambda is wrapped in explicit parentheses
- Checks the flag in mergeMultiParamLambdas to skip parenthesized lambdas

This correctly parses:
- `arrayFold(acc,x -> body, arr, init)` - merges acc into lambda params
- `delay(time, (time -> 0.5), ...)` - keeps time as separate argument

Fixes all 17 statements in 02718_array_fold test and 3 in 02418_aggregate_combinators.

Author: claude

ast/ast.go

@@ -1440,9 +1440,10 @@ func (t *TupleAccess) expressionNode()     {}
 
 // Lambda represents a lambda expression.
 type Lambda struct {
-	Position   token.Position `json:"-"`
-	Parameters []string       `json:"parameters"`
-	Body       Expression     `json:"body"`
+	Position      token.Position `json:"-"`
+	Parameters    []string       `json:"parameters"`
+	Body          Expression     `json:"body"`
+	Parenthesized bool           `json:"-"` // True if wrapped in explicit parentheses
 }
 
 func (l *Lambda) Pos() token.Position { return l.Position }

parser/expression.go

@@ -158,6 +158,58 @@ func (p *Parser) parseFunctionArgumentList() []ast.Expression {
 		}
 	}
 
+	// Post-process: merge consecutive identifiers followed by a lambda into a multi-param lambda
+	// Pattern: [Ident("acc"), Lambda(["x"], body)] -> [Lambda(["acc", "x"], body)]
+	exprs = mergeMultiParamLambdas(exprs)
+
+	return exprs
+}
+
+// mergeMultiParamLambdas looks for pattern [Ident, Ident, ..., Lambda] at the START
+// of the expression list and merges them into a single multi-param lambda.
+// This handles ClickHouse's syntax: acc,x -> body (multi-param lambda without parentheses)
+// This ONLY applies at position 0 - identifiers in the middle are regular arguments.
+func mergeMultiParamLambdas(exprs []ast.Expression) []ast.Expression {
+	if len(exprs) < 2 {
+		return exprs
+	}
+
+	// Only check at position 0 - the pattern must start at the beginning
+	if ident, ok := exprs[0].(*ast.Identifier); ok && len(ident.Parts) == 1 {
+		// Count consecutive simple identifiers at the start
+		j := 0
+		var params []string
+		for j < len(exprs) {
+			if id, ok := exprs[j].(*ast.Identifier); ok && len(id.Parts) == 1 {
+				params = append(params, id.Name())
+				j++
+			} else {
+				break
+			}
+		}
+		// Check if the next expression is a lambda and we have at least one identifier
+		if j < len(exprs) && len(params) >= 1 {
+			if lambda, ok := exprs[j].(*ast.Lambda); ok {
+				// Don't merge if lambda was explicitly parenthesized
+				// e.g., f(a, (x -> y)) should NOT merge 'a' into the lambda
+				if lambda.Parenthesized {
+					return exprs
+				}
+				// Merge the identifiers into the lambda's parameters
+				newParams := make([]string, 0, len(params)+len(lambda.Parameters))
+				newParams = append(newParams, params...)
+				newParams = append(newParams, lambda.Parameters...)
+				lambda.Parameters = newParams
+				// Return lambda followed by remaining expressions
+				result := make([]ast.Expression, 0, len(exprs)-j)
+				result = append(result, lambda)
+				result = append(result, exprs[j+1:]...)
+				return result
+			}
+		}
+	}
+
+	// No merge needed
 	return exprs
 }
 
@@ -1019,6 +1071,13 @@ func (p *Parser) parseGroupedOrTuple() ast.Expression {
 		binExpr.Parenthesized = true
 	}
 
+	// Mark lambda expressions as parenthesized so we don't merge them
+	// with preceding identifiers in multi-param lambda detection
+	// e.g., f(a, (x -> y)) should NOT merge 'a' into the lambda
+	if lambda, ok := first.(*ast.Lambda); ok {
+		lambda.Parenthesized = true
+	}
+
 	return first
 }
 

parser/testdata/02418_aggregate_combinators/metadata.json

@@ -1,7 +1 @@
-{
-  "explain_todo": {
-    "stmt11": true,
-    "stmt16": true,
-    "stmt28": true
-  }
-}
+{}

parser/testdata/02718_array_fold/metadata.json

@@ -1,21 +1 @@
-{
-  "explain_todo": {
-    "stmt10": true,
-    "stmt12": true,
-    "stmt13": true,
-    "stmt14": true,
-    "stmt15": true,
-    "stmt16": true,
-    "stmt17": true,
-    "stmt18": true,
-    "stmt20": true,
-    "stmt21": true,
-    "stmt22": true,
-    "stmt27": true,
-    "stmt34": true,
-    "stmt6": true,
-    "stmt7": true,
-    "stmt8": true,
-    "stmt9": true
-  }
-}
+{}