Iain,
this is the first of 2 patches I needed on top of your github WIP 
series.  It is against GCC-10, but AFAICT will also be needed on trunk.

We were not descending into the object operand of COMPONENT_REF exprs, 
and end up bitcopying a field out of a temporary, leading to badness. 
The breaking out of replace_unary_preamble is merely coincidental 
simplification.

I also think one needs to descend the second operand of COMPOUND_EXPR, 
but when I did that badness happens, probably because of the artificial 
COMPOUND_EXPRs that get generated, which you mentioned?

The bug manifested itself in Folly, and it was too difficult to extract 
a testcase.

nathan
-- 
Nathan Sidwell