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From: Eric Botcazou <botcazou@adacore.com>
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Subject: Re: qual_union_type help
Date: Wed, 23 Feb 2022 19:19:18 +0100
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> That makes sense during construction we also know what the value of
> the discriminant is. What does the Ada front-end replace the
> placeholder_exprs with? Can it simply be the value of the discriminant
> at constructor? I haven't tried that.

Ultimately all PLACEHOLDER_EXPRs need to be replaced by something in the code, 
i.e. they can only survive in (abstract) types.  There is an entire machinery 
in tree.c for that, called both from the front-end and middle-end in Ada.  You 
can replace it with an explicit value (SUBSTITUTE_IN_EXPR) or you can search 
for it in an object (SUBSTITUTE_PLACEHOLDER_IN_EXPR).  You can presumably do 
it through the gimplification hook.

-- 
Eric Botcazou