diff --git a/gcc/c/c-parser.cc b/gcc/c/c-parser.cc
index f1aaab12e6e..df44a915ed4 100644
--- a/gcc/c/c-parser.cc
+++ b/gcc/c/c-parser.cc
@@ -3222,7 +3222,7 @@ c_parser_declaration_or_fndef (c_parser *parser, bool fndef_ok,
       else
 	fnbody = c_parser_compound_statement (parser, &endloc);
       tree fndecl = current_function_decl;
-      if (nested)
+      if (nested && specs->declspec_il == cdil_none)
 	{
 	  tree decl = current_function_decl;
 	  /* Mark nested functions as needing static-chain initially.
@@ -3235,6 +3235,15 @@ c_parser_declaration_or_fndef (c_parser *parser, bool fndef_ok,
 	  c_pop_function_context ();
 	  add_stmt (build_stmt (DECL_SOURCE_LOCATION (decl), DECL_EXPR, decl));
 	}
+      else if (nested)
+	{
+	  if (specs->declspec_il == cdil_rtl)
+	    error ("%<__RTL%> function cannot be a nested function");
+	  else
+	    error ("%<__GIMPLE%> function cannot be a nested function");
+	  finish_function (endloc);
+	  c_pop_function_context ();
+	}
       else
 	{
 	  if (fnbody)
diff --git a/gcc/testsuite/gcc.dg/gimplefe-error-16.c b/gcc/testsuite/gcc.dg/gimplefe-error-16.c
new file mode 100644
index 00000000000..4cdeac889c8
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/gimplefe-error-16.c
@@ -0,0 +1,10 @@
+/* { dg-do compile } */
+/* { dg-options "-fgimple" } */
+/* PR c/121421 */
+/* Gimple functions cannot be nested functions. */
+
+
+void main(void)
+{
+    void __GIMPLE b(){} /* { dg-error "cannot be a nested" } */
+}