[TCLCORE] Tcl 9 `expr` bug, int.rep typing

[Andreas K. <and...@gm...>]

Hi all.

I believe that I have found a bug in the `expr` command of

	Tcl 9 (commit <1f19a433ec>).

This issue is, IMHO, __not__ a release blocker.

It should not affect the Tcl level at all.

Note however that it can trip/break scripts using extensions
performing type-checks while not doing or able to do shimmering.

IOW extensions expecting expect specific int-reps. Among the affected
will be extensions providing objtypes do not have a string-rep, making
shimmering impossible.

Other extensions will simply shimmer the value back to the int-rep
they need. This is why the script level is not affected, except for
a performance loss I did not bother to try and measure.


Test code:
```
proc show-type {x} { puts <>=[::tcl::unsupported::representation $x] }

set i xx
show-type $i
show-type [expr {0 ? $i : $i}]
show-type [expr {$i}]
```

For Tcl 8.6 the results are:

```
<>=value is a pure string with a refcount of 4, object pointer at 0x1ec87b0, 
string representation "xx"
<>=value is a pure string with a refcount of 3, object pointer at 0x1ec9440, 
string representation "xx"
<>=value is a pure string with a refcount of 3, object pointer at 0x1ecacd0, 
string representation "xx"
```

For Tcl 9 <commit 1f19a433ec> the results for the `expr` command change:

```
<>=value is a pure string with a refcount of 4, object pointer at 
0x55dc2b5776d0, string representation "xx"
<>=value is a list with a refcount of 3, object pointer at 0x55dc2b52aab0, 
internal representation 0x55dc2b5a30a0:0x0, string representation "xx"
<>=value is a list with a refcount of 3, object pointer at 0x55dc2b578c00, 
internal representation 0x55dc2b5a30a0:0x0, string representation "xx"
```

It looks as if the $x going into `expr` is wrapped into a single-element list.
Something I truly see no reason for.

-- 
Happy Tcling,
	Andreas Kupries <and...@gm...>
			<https://core.tcl-lang.org/akupries/>
			<https://akupries.tclers.tk/>
	Developer @	SUSE Software Solutions Germany GmbH
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