Specialize Guarded Pattern Parsing for Cases and Catches

Catch patterns are allowed to forgo their pattern, while cases must have patterns. Since these productions have two different types, split these two distinct productions apart.

Fixes #1040
rdar://101815618

Author: CodaFi

Sources/SwiftParser/Statements.swift

@@ -490,7 +490,7 @@ extension Parser {
       var keepGoing: RawTokenSyntax? = nil
       var loopProgress = LoopProgressCondition()
       repeat {
-        let (pattern, whereClause) = self.parseGuardedPattern(.catch)
+        let (pattern, whereClause) = self.parseGuardedCatchPattern()
         keepGoing = self.consume(if: .comma)
         catchItems.append(RawCatchItemSyntax(
           pattern: pattern, whereClause: whereClause, trailingComma: keepGoing,
@@ -505,6 +505,38 @@ extension Parser {
       body: body,
       arena: self.arena)
   }
+
+  /// Parse a pattern-matching clause for a catch statement,
+  /// including the guard expression.
+  ///
+  /// Grammar
+  /// =======
+  ///
+  ///     catch-pattern → pattern where-clause?
+  private mutating func parseGuardedCatchPattern() -> (RawPatternSyntax?, RawWhereClauseSyntax?) {
+    // If this is a 'catch' clause and we have "catch {" or "catch where...",
+    // then we get an implicit "let error" pattern.
+    let pattern: RawPatternSyntax?
+    if self.at(any: [ .leftBrace, .whereKeyword ]) {
+      pattern = nil
+    } else {
+      pattern = self.parseMatchingPattern(context: .matching)
+    }
+
+    // Parse the optional 'where' guard.
+    let whereClause: RawWhereClauseSyntax?
+    if let whereKeyword = self.consume(if: .whereKeyword) {
+      let guardExpr = self.parseExpression(.basic)
+      whereClause = RawWhereClauseSyntax(
+        whereKeyword: whereKeyword,
+        guardResult: guardExpr,
+        arena: self.arena
+      )
+    } else {
+      whereClause = nil
+    }
+    return (pattern, whereClause)
+  }
 }
 
 // MARK: Iteration Statements
@@ -821,7 +853,7 @@ extension Parser {
       var keepGoing: RawTokenSyntax? = nil
       var loopProgress = LoopProgressCondition()
       repeat {
-        let (pattern, whereClause) = self.parseGuardedPattern(.case)
+        let (pattern, whereClause) = self.parseGuardedCasePattern()
         keepGoing = self.consume(if: .comma)
         caseItems.append(RawCaseItemSyntax(
           pattern: pattern, whereClause: whereClause, trailingComma: keepGoing,
@@ -859,39 +891,21 @@ extension Parser {
     )
   }
 
-  enum GuardedPatternContext {
-    case `case`
-    case `catch`
-  }
-
-  /// Parse a pattern-matching clause for a case or catch statement,
+  /// Parse a pattern-matching clause for a case statement,
   /// including the guard expression.
   ///
   /// Grammar
   /// =======
   ///
   ///     case-item     → pattern where-clause?
-  ///     catch-pattern → pattern where-clause?
-  mutating func parseGuardedPattern(
-    _ context: GuardedPatternContext
-  ) -> (RawPatternSyntax, RawWhereClauseSyntax?) {
-    let flavor: ExprFlavor
-    switch context {
-    // 'case' is terminated with a colon and so allows a trailing closure.
-    case .`case`:
-      flavor = .trailingClosure
-    // 'catch' is terminated with a brace and so cannot.
-    case .`catch`:
-      flavor = .basic
-    }
-
+  mutating func parseGuardedCasePattern() -> (RawPatternSyntax, RawWhereClauseSyntax?) {
     let pattern = self.parseMatchingPattern(context: .matching)
 
     // Parse the optional 'where' guard, with this particular pattern's bound
     // vars in scope.
     let whereClause: RawWhereClauseSyntax?
     if let whereKeyword = self.consume(if: .whereKeyword) {
-      let guardExpr = self.parseExpression(flavor)
+      let guardExpr = self.parseExpression(.trailingClosure)
       whereClause = RawWhereClauseSyntax(
         whereKeyword: whereKeyword,
         guardResult: guardExpr,

Tests/SwiftParserTest/StatementTests.swift

@@ -105,6 +105,12 @@ final class StatementTests: XCTestCase {
        }
        """
     )
+
+    AssertParse(
+      """
+      do {}
+      catch where (error as NSError) == NSError() {}
+      """)
   }
 
   func testReturn() {