Powershell: The null-coalescing operator evaluates the left-hand operand twice.

[chtoucas]

Steps to reproduce

function A {
    Write-Host "In A"
    return "something"
}

# Here, A is called twice
$y = (A) ?? "blabla"

Expected behavior

The function A should be called only once.

In A

Actual behavior

The function A is called twice.

In A
In A

Environment data

Name                           Value
----                           -----
PSVersion                      7.0.0
PSEdition                      Core
GitCommitId                    7.0.0
OS                             Microsoft Windows 10.0.18362
Platform                       Win32NT
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0…}
PSRemotingProtocolVersion      2.3
SerializationVersion           1.1.0.1
WSManStackVersion              3.0

[iSazonov]

/cc @adityapatwardhan @rjmholt

[SeeminglyScience]

The LHS here needs to be saved to a temporary variable:

https://github.com/PowerShell/PowerShell/blob/42248a072f806586a262eb2821ac81cf8ed62d09/src/System.Management.Automation/engine/parser/Compiler.cs#L991-L997

Also

https://github.com/PowerShell/PowerShell/blob/42248a072f806586a262eb2821ac81cf8ed62d09/src/System.Management.Automation/engine/parser/Compiler.cs#L985-L988

AutomationNull is a static class that stores an instance of PSObject in a static field. This check will never be true.

[SeeminglyScience]

/cc @SteveL-MSFT for visibility. Double execution could cause some very dangerous side effects. May be a good candidate for inclusion in a servicing update.

[msftbot[bot]]

:tada:This issue was addressed in #12667, which has now been successfully released as v7.0.2.:tada:

Handy links:

• Release Notes

[msftbot[bot]]

:tada:This issue was addressed in #12667, which has now been successfully released as v7.1.0-preview.4.:tada:

Handy links:

• Release Notes