[Sema] Move anonymous closure argument check from parser to semantic analysis

include/swift/AST/DiagnosticsParse.def

@@ -1311,8 +1311,6 @@ ERROR(dollar_identifier_decl,none,
      "cannot declare entity named %0; the '$' prefix is reserved for "
      "implicitly-synthesized declarations", (Identifier))
 
-ERROR(anon_closure_arg_not_in_closure,none,
-      "anonymous closure argument not contained in a closure", ())
 ERROR(anon_closure_arg_in_closure_with_args,none,
       "anonymous closure arguments cannot be used inside a closure that has "
       "explicit arguments", ())

include/swift/AST/DiagnosticsSema.def

@@ -1702,6 +1702,8 @@ ERROR(extra_trailing_closure_in_call,none,
 ERROR(closure_bad_param,none,
       "%select{|trailing }1closure passed to parameter of type %0 that does not "
       "accept a closure", (Type, bool))
+ERROR(anon_closure_arg_not_in_closure,none,
+      "anonymous closure argument not contained in a closure", ())
 GROUPED_WARNING(unlabeled_trailing_closure_deprecated,
         TrailingClosureMatching,Deprecation,
         "backward matching of the unlabeled trailing closure is deprecated; label the argument with %0 to suppress this warning",

lib/Parse/ParseExpr.cpp

@@ -3192,14 +3192,10 @@ Expr *Parser::parseExprAnonClosureArg() {
   auto closure = dyn_cast_or_null<ClosureExpr>(
       dyn_cast<AbstractClosureExpr>(CurDeclContext));
   if (!closure) {
-    if (Context.LangOpts.DebuggerSupport) {
-      auto refKind = DeclRefKind::Ordinary;
-      auto identifier = Context.getIdentifier(Name);
-      return new (Context) UnresolvedDeclRefExpr(DeclNameRef(identifier),
-                                                 refKind, DeclNameLoc(Loc));
-    }
-    diagnose(Loc, diag::anon_closure_arg_not_in_closure);
-    return new (Context) ErrorExpr(Loc);
+    auto refKind = DeclRefKind::Ordinary;
+    auto identifier = Context.getIdentifier(Name);
+    return new (Context) UnresolvedDeclRefExpr(DeclNameRef(identifier), refKind,
+                                               DeclNameLoc(Loc));
   }
 
   // Check whether the closure already has explicit parameters.

lib/Sema/PreCheckTarget.cpp

@@ -946,6 +946,23 @@ Expr *TypeChecker::resolveDeclRefExpr(UnresolvedDeclRefExpr *UDRE,
   }
 
   auto emitBasicError = [&] {
+    // Check for anonymous closure arguments (e.g. $0, $1) used outside
+    // closures.
+    if (Name.isSimpleName()) {
+      StringRef nameStr = Name.getBaseIdentifier().str();
+      if (nameStr.starts_with("$")) {
+        StringRef numStr = nameStr.substr(1);
+        unsigned ArgNo;
+        if (!numStr.getAsInteger(10, ArgNo)) {
+          auto *closure = dyn_cast_or_null<ClosureExpr>(DC);
+          if (!closure) {
+            Context.Diags.diagnose(Loc, diag::anon_closure_arg_not_in_closure)
+                .highlight(UDRE->getSourceRange());
+            return;
+          }
+        }
+      }
+    }
     if (Name.isSimpleName(Context.Id_self)) {
       // `self` gets diagnosed with a different error when it can't be found.
       Context.Diags.diagnose(Loc, diag::cannot_find_self_in_scope)

test/Sema/issue-54030.swift

@@ -0,0 +1,9 @@
+// RUN: %target-typecheck-verify-swift
+
+// https://github.com/swiftlang/swift/issues/54030
+
+if $0 {} // expected-error {{anonymous closure argument not contained in a closure}}
+
+#if false
+if $0 {}
+#endif