One clue seems to be this.  In semantic-analyze-possible-completions and semantic-analyze-possible-completions-default in complete.el, the argument to the `context' parameter seems to be missing a value in the :prefixtypes slot, when completing on the variable `s' in  It then bails out of semantic-analyze-possible-completions-default beginning at line 144. 

Question:  where does the argument to `context' come from when invoking semantic-analyze-possible-completions, say, interactively?