Add missing switch expression error

Author: kimdv

Sources/SwiftParser/Expressions.swift

@@ -2222,7 +2222,16 @@ extension Parser {
   ) -> RawSwitchExprSyntax {
     let (unexpectedBeforeSwitchKeyword, switchKeyword) = self.eat(switchHandle)
 
-    let subject = self.parseExpression(.basic)
+    // If there is no expression, like `switch { default: return false }` then left brace would parsed as
+    // a `RawClosureExprSyntax` in the condition, which is most likely not what the user meant.
+    // Create a missing condition instead and use the `{` for the start of the body.
+    let subject: RawExprSyntax
+    if self.at(.leftBrace) {
+      subject = RawExprSyntax(RawMissingExprSyntax(arena: self.arena))
+    } else {
+      subject = self.parseExpression(.basic)
+    }
+
     let (unexpectedBeforeLBrace, lbrace) = self.expect(.leftBrace)
 
     let cases = self.parseSwitchCases(allowStandaloneStmtRecovery: !lbrace.isMissing)

Sources/SwiftParserDiagnostics/ParseDiagnosticsGenerator.swift

@@ -1043,6 +1043,18 @@ public class ParseDiagnosticsGenerator: SyntaxAnyVisitor {
     return .visitChildren
   }
 
+  public override func visit(_ node: SwitchExprSyntax) -> SyntaxVisitorContinueKind {
+    if shouldSkip(node) {
+      return .skipChildren
+    }
+
+    if node.expression.is(MissingExprSyntax.self) && !node.cases.isEmpty {
+      addDiagnostic(node.expression, .missingExpressionInSwitchStatement, handledNodes: [node.expression.id])
+    }
+
+    return .visitChildren
+  }
+
   public override func visit(_ node: SwitchCaseSyntax) -> SyntaxVisitorContinueKind {
     if shouldSkip(node) {
       return .skipChildren

Sources/SwiftParserDiagnostics/ParserDiagnosticMessages.swift

@@ -164,6 +164,9 @@ extension DiagnosticMessage where Self == StaticParserError {
   public static var missingConformanceRequirement: Self {
     .init("expected ':' or '==' to indicate a conformance or same-type requirement")
   }
+  public static var missingExpressionInSwitchStatement: Self {
+    .init("expected expression in 'switch' statement")
+  }
   public static var misspelledAsync: Self {
     .init("expected async specifier; did you mean 'async'?")
   }

Tests/SwiftParserTest/translated/RecoveryTests.swift

@@ -621,82 +621,84 @@ final class RecoveryTests: XCTestCase {
   func testRecovery46() {
     assertParse(
       """
-      switch {
-      }1️⃣
+      switch 1️⃣{
+      }
       """,
       diagnostics: [
-        // TODO: Old parser expected error on line 1: expected expression in 'switch' statement
-        // TODO: Old parser expected error on line 1: 'switch' statement body must have at least one 'case' or 'default' block
-        DiagnosticSpec(message: "expected '{}' in 'switch' statement")
+        DiagnosticSpec(locationMarker: "1️⃣", message: "expected expression in 'switch' statement")
       ]
     )
   }
 
   func testRecovery47() {
     assertParse(
       """
-      switch
+      switch 1️⃣
       {
-      }1️⃣
+      }
       """,
       diagnostics: [
-        // TODO: Old parser expected error on line 1: expected expression in 'switch' statement
-        // TODO: Old parser expected error on line 1: 'switch' statement body must have at least one 'case' or 'default' block
-        DiagnosticSpec(message: "expected '{}' in 'switch' statement")
+        DiagnosticSpec(locationMarker: "1️⃣", message: "expected expression in 'switch' statement")
       ]
     )
   }
 
   func testRecovery48() {
     assertParse(
       """
-      switch {
-        1️⃣case _: return
-      }2️⃣
+      switch 1️⃣{
+        2️⃣case _: return
+      }
       """,
       diagnostics: [
-        // TODO: Old parser expected error on line 1: expected expression in 'switch' statement
-        DiagnosticSpec(locationMarker: "1️⃣", message: "'case' can only appear inside a 'switch' statement or 'enum' declaration"),
-        DiagnosticSpec(locationMarker: "2️⃣", message: "expected '{}' in 'switch' statement"),
+        DiagnosticSpec(locationMarker: "1️⃣", message: "expected expression in 'switch' statement")
       ]
     )
   }
 
   func testRecovery49() {
     assertParse(
       """
-      switch {
-        1️⃣case Int: return
-        2️⃣case _: return
-      }3️⃣
+      switch 1️⃣{
+        case Int: return
+        case _: return
+      }
       """,
       diagnostics: [
-        // TODO: Old parser expected error on line 1: expected expression in 'switch' statement
+        DiagnosticSpec(locationMarker: "1️⃣", message: "expected expression in 'switch' statement")
         // TODO: Old parser expected error on line 2: 'is' keyword required to pattern match against type name, Fix-It replacements: 10 - 10 = 'is '
-        DiagnosticSpec(locationMarker: "1️⃣", message: "'case' can only appear inside a 'switch' statement or 'enum' declaration"),
-        DiagnosticSpec(locationMarker: "2️⃣", message: "'case' can only appear inside a 'switch' statement or 'enum' declaration"),
-        DiagnosticSpec(locationMarker: "3️⃣", message: "expected '{}' in 'switch' statement"),
       ]
     )
   }
 
   func testRecovery50() {
     assertParse(
       """
-      switch { 42 } {
-        case _: return
+      switch 1️⃣{ 2️⃣42 } {
+        3️⃣case _: return
       }
-      """
+      """,
+      diagnostics: [
+        DiagnosticSpec(locationMarker: "1️⃣", message: "expected expression in 'switch' statement"),
+        DiagnosticSpec(locationMarker: "2️⃣", message: "all statements inside a switch must be covered by a 'case' or 'default' label"),
+        DiagnosticSpec(locationMarker: "3️⃣", message: "'case' can only appear inside a 'switch' statement or 'enum' declaration"),
+      ]
     )
   }
 
   func testRecovery51() {
     assertParse(
       """
-      switch { 42 }() {
-        case _: return
+      switch 1️⃣{ 2️⃣42 }()3️⃣ {
+        4️⃣case _: return
       }
-      """
+      """,
+      diagnostics: [
+        DiagnosticSpec(locationMarker: "1️⃣", message: "expected expression in 'switch' statement"),
+        DiagnosticSpec(locationMarker: "2️⃣", message: "all statements inside a switch must be covered by a 'case' or 'default' label"),
+        DiagnosticSpec(locationMarker: "3️⃣", message: "consecutive statements on a line must be separated by ';'"),
+        DiagnosticSpec(locationMarker: "4️⃣", message: "'case' can only appear inside a 'switch' statement or 'enum' declaration"),
+      ]
     )
   }