Variable value if exception is raised

[nytpu <alex@nytpu.com>]

Hello everyone,

If an exception is *explicitly* raised during a variable assignment, what 
happens to the variable contents?  Are they in an undefined ("abnormal") 
state, or are the previous contents preserved?

For example:
```
with Ada.Text_IO;
procedure Test is
    function Always_Raises return Integer is
    begin
        raise Program_Error;
        return 1;
    end Always_Raises;
    
    I : Integer := 0;
begin
    -- insert a nested handler, because the ARM § 11.4 ¶ 3 *does*
    -- say that the currently executing body is "abnormally
    -- completed" (including finalizing everything) before
    -- entering the exception handler
    begin
        I := Always_Raises;
    exception
        when others => null;
    end;
    Ada.Text_IO.Put_Line(Integer'Image(I));
end;
```
What, if anything, will be printed?  (Disclaimer: I know the preexisting 
variable value will be preserved in GNAT specifically, but I'm asking if 
the standard guarantees that's the case)

I read through the ARM 2012 § 11 and § 5.2, as well as skimming through 
everything related to “assignment” and “exceptions” in the ARM index;
and didn't see much relating to this.  All I saw is this:
> When an exception occurrence is raised by the execution of a given 
> construct, the rest of the execution of that construct is abandoned
— ARM 2012 § 11.4 ¶ 3
Which I guess implicitly protects variable values since assigning to a 
variable is performed after evaluating the right hand side, but still not 
necessarily a clear answer.

I did see in § 13.9.1 that language-defined validity checks (e.g. bounds 
checks) failing or calling `abort` in a task during an assignment will 
cause the variable to enter an "abnormal" (i.e. invalid) state, but that 
doesn't cover user-raised exceptions.

-- 
~nytpu