ExecMakeTableFunctionResult vs. pre-evaluated functions

I've spent today messing with making the planner substitute inline
definitions of simple SQL functions, per the comment in
src/backend/optimizer/util/clauses.c:
* XXX Possible future improvement: if the func is SQL-language, and its* definition is simply "SELECT expression", we
couldparse and substitute* the expression here.  This would avoid much runtime overhead, and perhaps* expose
opportunitiesfor constant-folding within the expression even if* not all the func's input args are constants.  It'd be
appropriateto do* that here, not in the parser, since we wouldn't want it to happen until* after rule
substitution/rewriting.

It seems to work 99%, but I'm seeing this failure in the regression
tests:
 CREATE FUNCTION getfoo(int) RETURNS int AS 'SELECT $1;' LANGUAGE SQL; SELECT * FROM getfoo(1) AS t1;
! ERROR:  ExecMakeTableFunctionResult: expression is not a function call

which of course happens because the table-function expression has been
reduced to just a constant "1" by the time the executor sees it.

A grotty answer is to not apply constant-expression folding to table
function RTE entries.  A better answer would be to make
ExecMakeTableFunctionResult more flexible, but I'm not quite sure what
it should do if presented a non-function-call expression tree.  Any
thoughts?
        regards, tom lane

PS: another little problem isregression=# explain SELECT * FROM getfoo(1) AS t1;server closed the connection
unexpectedly
but I'm sure that's just a lack of flexibility in explain.c ...