Fix pr 17203, disallow creating pre-existing parameters with guile

diff --git a/gdb/guile/scm-param.c b/gdb/guile/scm-param.c
index 5bfb4ae..a88e8bc 100644
--- a/gdb/guile/scm-param.c
+++ b/gdb/guile/scm-param.c
@@ -966,9 +966,23 @@  gdbscm_make_parameter (SCM name_scm, SCM rest)
   return p_scm;
 }
 
+/* Subroutine of gdbscm_register_parameter_x to simplify it.
+   Return non-zero if parameter NAME is already defined in LIST.  */
+
+static int
+pascm_parameter_defined_p (const char *name, struct cmd_list_element *list)
+{
+  struct cmd_list_element *c;
+
+  c = lookup_cmd_1 (&name, list, NULL, 1);
+
+  /* If the name is ambiguous that's ok, it's a new parameter still.  */
+  return c != NULL && c != CMD_LIST_AMBIGUOUS;
+}
+
 /* (register-parameter! <gdb:parameter>) -> unspecified
 
-   It is an error to register a parameter more than once.  */
+   It is an error to register a pre-existing parameter.  */
 
 static SCM
 gdbscm_register_parameter_x (SCM self)
@@ -990,6 +1004,17 @@  gdbscm_register_parameter_x (SCM self)
   p_smob->cmd_name = gdbscm_gc_xstrdup (cmd_name);
   xfree (cmd_name);
 
+  if (pascm_parameter_defined_p (p_smob->cmd_name, *set_list))
+    {
+      gdbscm_misc_error (FUNC_NAME, SCM_ARG1, self,
+		_("parameter exists, \"set\" command is already defined"));
+    }
+  if (pascm_parameter_defined_p (p_smob->cmd_name, *show_list))
+    {
+      gdbscm_misc_error (FUNC_NAME, SCM_ARG1, self,
+		_("parameter exists, \"show\" command is already defined"));
+    }
+
   TRY_CATCH (except, RETURN_MASK_ALL)
     {
       add_setshow_generic (p_smob->type, p_smob->cmd_class,
diff --git a/gdb/testsuite/gdb.guile/scm-parameter.exp b/gdb/testsuite/gdb.guile/scm-parameter.exp
index 0dd8a47..c0df185 100644
--- a/gdb/testsuite/gdb.guile/scm-parameter.exp
+++ b/gdb/testsuite/gdb.guile/scm-parameter.exp
@@ -166,3 +166,31 @@  with_test_prefix "test-restricted-param" {
     gdb_test "set test-restricted-param 42" "Error: Range of parameter is 0-10."
     gdb_test "show test-restricted-param" "The value of the restricted parameter is 2."
 }
+
+# Test registering a parameter that already exists.
+
+gdb_test "guile (register-parameter! (make-parameter \"height\"))" \
+    "ERROR.*is already defined.*" "error registering existing parameter"
+
+# Test registering a parameter named with what was an ambiguous spelling
+# of existing parameters.
+
+gdb_test_multiline "previously ambiguously named boolean parameter" \
+    "guile" "" \
+    "(define prev-ambig" "" \
+    "  (make-parameter \"print s\"" "" \
+    "   #:parameter-type PARAM_BOOLEAN))" "" \
+    "end"
+
+gdb_test_no_output "guile (register-parameter! prev-ambig)"
+
+with_test_prefix "previously-ambiguous" {
+    gdb_test "guile (print (parameter-value prev-ambig))" "= #f" "parameter value (false)"
+    gdb_test "show print s" "Command is not documented is off." "Show parameter off"
+    gdb_test_no_output "set print s on"
+    gdb_test "show print s" "Command is not documented is on." "Show parameter on"
+    gdb_test "guile (print (parameter-value prev-ambig))" "= #t" "parameter value (true)"
+    gdb_test "help show print s" "This command is not documented." "show help"
+    gdb_test "help set print s" "This command is not documented." "set help"
+    gdb_test "help set print" "set print s -- This command is not documented.*" "general help"
+}