Fix parsing of "return <macro-expansion>"

Author: DougGregor

Sources/SwiftParser/Declarations.swift

@@ -32,9 +32,16 @@ extension TokenConsumer {
   mutating func atStartOfDeclaration(
     isAtTopLevel: Bool = false,
     allowInitDecl: Bool = true,
-    allowRecovery: Bool = false
+    allowRecovery: Bool = false,
+    preferPoundAsExpression: Bool = false
   ) -> Bool {
     if self.at(anyIn: PoundDeclarationStart.self) != nil {
+      // If we are in a context where we prefer # to be an expression,
+      // treat it as one if it's not at the start of the line.
+      if preferPoundAsExpression && self.at(.pound) && !self.currentToken.isAtStartOfLine {
+        return false
+      }
+
       return true
     }
 

Sources/SwiftParser/Statements.swift

@@ -985,7 +985,8 @@ extension Parser {
       .poundIfKeyword, .poundErrorKeyword, .poundWarningKeyword,
       .poundEndifKeyword, .poundElseKeyword, .poundElseifKeyword,
     ])
-      && !self.atStartOfStatement() && !self.atStartOfDeclaration()
+      && !self.atStartOfStatement() &&
+        !self.atStartOfDeclaration(preferPoundAsExpression: true)
     {
       let parsedExpr = self.parseExpression()
       if hasMisplacedTry && !parsedExpr.is(RawTryExprSyntax.self) {

Tests/SwiftParserTest/ExpressionTests.swift

@@ -726,7 +726,7 @@ final class ExpressionTests: XCTestCase {
       "#keyPath((b:1️⃣)2️⃣",
       diagnostics: [
         DiagnosticSpec(locationMarker: "1️⃣", message: "expected value in tuple"),
-        DiagnosticSpec(locationMarker: "2️⃣", message: "expected ')' to end pound literal expression"),
+        DiagnosticSpec(locationMarker: "2️⃣", message: "expected ')' to end pound literal declaration"),
       ]
     )
   }