I think these should be invalid:

for (let.foo of bar);
for (let().bar of baz);
for (let``.bar of baz);

ForInOfStatement:

for ( [lookahead ≠ let] LeftHandSideExpression[?Yield, ?Await] of AssignmentExpression[+In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return]

Expressions starting with ( are ok, like for ((let).foo of bar);, for ((let.foo) of bar);, for ((let) of bar);, and of course declarations like for (let foo of bar);, for (let [foo] of bar);