Reduce recovery precedence of arrow in function type to exprKeyword

Only allow recovery to the arrow with exprKeyword precedence so we only skip over misplaced identifiers and don't e.g. recover to an errow in a 'where' clause.

rdar://106457182

Author: ahoppen

Sources/SwiftParser/Declarations.swift

@@ -1376,7 +1376,9 @@ extension Parser {
 
     let output: RawReturnClauseSyntax?
 
-    if self.at(.arrow) || self.canRecoverTo(TokenSpec(.arrow, allowAtStartOfLine: false)) != nil {
+    /// Only allow recovery to the arrow with exprKeyword precedence so we only
+    /// skip over misplaced identifiers and don't e.g. recover to an errow in a 'where' clause.
+    if self.at(.arrow) || self.canRecoverTo(TokenSpec(.arrow, recoveryPrecedence: .exprKeyword)) != nil {
       output = self.parseFunctionReturnClause(effectSpecifiers: &effectSpecifiers, allowNamedOpaqueResultType: true)
     } else {
       output = nil

Tests/SwiftParserTest/DeclarationTests.swift

@@ -1531,6 +1531,17 @@ final class DeclarationTests: XCTestCase {
       """
     )
   }
+
+  func testWhereClauseWithFunctionType() {
+    // A function type in the where clause isn't semantically valid but its fine
+    // with the parser. Make sure we don't recover to the arrow to parse the
+    // function return type.
+    AssertParse(
+      """
+      func badTypeConformance3<T>(_: T) where (T) -> () : EqualComparable { }
+      """
+    )
+  }
 }
 
 extension Parser.DeclAttributes {