Tcl Source Code

The bug will probably not show up if the args list is 
constructed in a single frame. The code below uses an 
external proc to create each arg which are then lappended 
to the final list used, with or without a default value. 

The first version of tcl where this shows up is 8.6.9, 
when the trampoline code started to be used. I've tested 
every released version of tcl since, compiling from source.

The error occurs in 8.6.9,10,11 8.7a5 and 9.0a3.
The error does not occur in 8.6.8,7,6,5 8.5.19 or 8.4.20.

The main platform is Ubuntu: uname -a
5.11.0-37-generic #41-Ubuntu SMP x86_64 x86_64 x86_64 GNU/Linux

I tried following the process with gdb/VS Code and in working and
non-working versions, the objv bytes is 0x0 when the proc commands
starts. 

Here's what makes the bug go away:

In the helper proc or in the main proc call:
llength $argName on each arg.

In the main proc:
puts $argsList

If you catch the error with proc, you can still make the bug
go away using the above calls. If you just repeat the same call
the bug is still there.

I have not found a way to repeat the bug without a helper proc.

Here's the simplified code which shows the bug:

proc splitArg {argSpec {splitChar !} {ArrayName ""}} {

    set firstPart [lindex $argSpec 0]
    set first [string first $splitChar $firstPart]

    if {$first > -1} {
        set argName [string range $firstPart 0 [expr {$first - 1}]]
        set extra [string range $firstPart [expr {$first + 1}] end]
    }
    if {$ArrayName eq ""} {
        set ArrayName $argName 
    }
    upvar $ArrayName ArgNameArray 

    array set ArgNameArray [lindex $argSpec 1]
    return $argName
}

proc testProc {procName argSpecs body} {

    set argList [list] 
    foreach argSpec $argSpecs {
        if {[array exists specArray]} {
            array unset specArray
        }
        set argName [splitArg $argSpec ! specArray]
        if {[info exists specArray(default)]} {
            lappend argList [list $argName $specArray(default)]
        } else {
            lappend argList [list $argName]
        }

    }

    proc $procName $argList $body
}

testProc myproc {{arg1!type1 {default "It Works, no bug in "}}}\
    {puts -nonewline $arg1;puts tcl[info patchlevel]}

myproc

Fix is now merged to 8.7 and 9.0 branches as well.

A more concise script to reproduce the issue is:

proc p1 [list [list [expr {1 + 2}] default]] {}

The issue is that to determine the length of the string, TclCreateProc() was only checking the size of any existing string repesentation of a value without taking into the consideration that the string representation might not be current.

Fixed in [75a9777184741e05]

Bisecting locates the behavior change of this script to

https://core.tcl-lang.org/tcl/info/c9251294d9b8b14d

Assigning for further review.