(Fbeginning_of_line) [PROMPT_IN_BUFFER]: Set point to

end of prompt if applied to a mini-buffer and BEG reached.

src/cmds.c

@@ -163,7 +163,17 @@ If scan reaches end of buffer, stop there without error.")
   else
     CHECK_NUMBER (n, 0);
 
+#if !NO_PROMPT_IN_BUFFER
+  {
+    int pos = XFASTINT (Fline_beginning_position (n));
+    if (INTEGERP (current_buffer->minibuffer_prompt_length)
+	&& pos < XFASTINT (current_buffer->minibuffer_prompt_length))
+      pos = XFASTINT (current_buffer->minibuffer_prompt_length);
+    SET_PT (pos);
+  }
+#else
   SET_PT (XINT (Fline_beginning_position (n)));
+#endif
   return Qnil;
 }