Derive better name for methods with external method tables. (#41930)

Author: maleadt

src/interpreter.c

@@ -79,6 +79,7 @@ static jl_value_t *eval_methoddef(jl_expr_t *ex, interpreter_state *s)
 {
     jl_value_t **args = jl_array_ptr_data(ex->args);
 
+    // generic function definition
     if (jl_expr_nargs(ex) == 1) {
         jl_value_t **args = jl_array_ptr_data(ex->args);
         jl_sym_t *fname = (jl_sym_t*)args[0];

src/method.c

@@ -831,7 +831,7 @@ JL_DLLEXPORT jl_method_t* jl_method_def(jl_svec_t *argdata,
 
     // TODO: derive our debug name from the syntax instead of the type
     name = mt->name;
-    if (mt == jl_type_type_mt || mt == jl_nonfunction_mt) {
+    if (mt == jl_type_type_mt || mt == jl_nonfunction_mt || external_mt) {
         // our value for `name` is bad, try to guess what the syntax might have had,
         // like `jl_static_show_func_sig` might have come up with
         jl_datatype_t *dt = jl_first_argument_datatype(argtype);