Re: Elaboration_check For Instantiations

[dewar@merv.cs.nyu.edu (Robert Dewar)]

Charlie says

<<     Are you sure about that, Robert?  (Never fearing, he leaps back
into the fray.)  11.5(2) says that language-defined checks, which in-
clude Elaboration_check, have to be made at run time and 3.11(14) says
that Program_error is raised if Elaboration_check fails.

     Of course, 11.5(1) says that pragma Suppress "gives permission to
an implementation" to omit the check, so the GNAT implementation is
technically correct, at least.>>


You are making a common mistake, GNAT is entirely correct. Detecting a check
that fails at compile time and compiling the appropriate raise is of course
completely permissible, and indeed highly desirable (note that annex H 
requires that a compiler that *does* detect such a situation at compile
time *must* output a warning).

Why is this correct? Becuase it is behaviorally equivalent to doing the
check at runtime. I often find that people do not understand the critical
as-if principle that applies to all compiler code generation. This is such
a case. Generating the raise of PE behaves *exactly* "as if" the check were
done at runtime and is therefore fine.

The whole point of 11.5(1) allowing you NOT to omit the check is precisely
to deal with cases like this, where it would take extra time and code to
omit the check. The purpose of Suppress is to permit the compiler to speed
up the code by omitting the check. It is *NOT* to guarantee that the 
exception will not be raised. 

So, yes, I am 100% sure!