Fix generic parameter round trip failure

Fixes #835
Resolves rdar://100321071

Author: ahoppen

Sources/SwiftParser/Declarations.swift

@@ -302,7 +302,7 @@ extension Parser {
         let attributes = self.parseAttributeList()
 
         let (unexpectedBeforeName, name) = self.expectIdentifier()
-        if unexpectedBeforeName == nil && name.isMissing && elements.isEmpty {
+        if attributes == nil && unexpectedBeforeName == nil && name.isMissing && elements.isEmpty {
           break
         }
 

Tests/SwiftParserTest/Declarations.swift

@@ -1059,6 +1059,21 @@ final class DeclarationTests: XCTestCase {
       }
       """)
   }
+
+  func testStandaloneAtSignInGenericParameter() {
+    AssertParse(
+      """
+      struct U<@#^DIAG^#
+      """,
+      diagnostics: [
+        DiagnosticSpec(message: "Expected name of attribute"),
+        DiagnosticSpec(message: "Expected identifier in generic parameter"),
+        DiagnosticSpec(message: "Expected '>' to end generic parameter clause"),
+        DiagnosticSpec(message: "Expected '{' to start struct"),
+        DiagnosticSpec(message: "Expected '}' to end struct"),
+      ]
+    )
+  }
 }
 
 extension Parser.DeclAttributes {