Don’t drop module selector in bad generic arg list

The code assumed that dropping a `MissingTypeSyntax` wouldn’t lose any tokens.

Author: beccadax

Sources/SwiftParser/Types.swift

@@ -448,7 +448,9 @@ extension Parser {
       repeat {
         let argument = self.parseGenericArgumentType()
 
-        if arguments.isEmpty, argument.raw.is(RawMissingTypeSyntax.self) {
+        if arguments.isEmpty, let missingType = argument.raw.as(RawMissingTypeSyntax.self),
+          missingType.moduleSelector == nil
+        {
           break
         }
 

Tests/SwiftParserTest/AttributeTests.swift

@@ -205,6 +205,33 @@ final class AttributeTests: ParserTestCase {
       experimentalFeatures: [.moduleSelector]
     )
 
+    // Found through mutation testing.
+    assertParse(
+      """
+      @objc<1️⃣::2️⃣:x::)
+      func f(_: Int, _: Int, _: Int, _: Int, _: Int) { }
+      """,
+      diagnostics: [
+        DiagnosticSpec(
+          message: "expected module name in module selector",
+          fixIts: ["insert module name"]
+        ),
+        DiagnosticSpec(
+          message: "expected type and '>' to end generic argument clause",
+          fixIts: ["insert type and '>'"]
+        ),
+        DiagnosticSpec(
+          locationMarker: "2️⃣",
+          message: "unexpected code ':x::)' in function"
+        ),
+      ],
+      fixedSource: """
+        @objc<<#identifier#>::<#type#>>:x::)
+        func f(_: Int, _: Int, _: Int, _: Int, _: Int) { }
+        """,
+      experimentalFeatures: [.moduleSelector]
+    )
+
     assertParse(
       """
       @objc(_:)