bind default values to `$PSBoundParameters`

method1.ps1

& {
    [CmdletBinding(DefaultParameterSetName = 'A')]
    param(
        [Parameter(ParameterSetName = 'A')]
        $ParamA = 'A',
        [Parameter(ParameterSetName = 'B')]
        $ParamB = 'B',
        [Parameter(Mandatory)]
        $ParamBoth
    )

    foreach ($set in $MyInvocation.MyCommand.ParameterSets) {
        if ($set.Name -ne $PSCmdlet.ParameterSetName) {
            continue
        }

        foreach ($param in $set.Parameters) {
            if ($PSBoundParameters.ContainsKey($param.Name)) {
                continue
            }
            if ([System.Management.Automation.PSCmdlet]::CommonParameters.Contains($param.Name)) {
                continue
            }
            if ([System.Management.Automation.PSCmdlet]::OptionalCommonParameters.Contains($param.Name)) {
                continue
            }
            
            $PSBoundParameters[$param.Name] = $ExecutionContext.SessionState.PSVariable.GetValue($param.Key)
        }
    }

    $PSBoundParameters
} -ParamBoth foo

method2.ps1

& {
    [CmdletBinding(DefaultParameterSetName = 'A')]
    param(
        [Parameter(ParameterSetName = 'A')]
        $ParamA = 'A',
        [Parameter(ParameterSetName = 'B')]
        $ParamB = 'B',
        [Parameter(Mandatory)]
        $ParamBoth
    )

    $enum = [System.Management.Automation.CommandMetadata]::new($MyInvocation.MyCommand).
        Parameters.GetEnumerator()

    foreach ($param in $enum) {
        if ($PSBoundParameters.ContainsKey($param.Key)) {
            continue
        }
        if (-not $param.Value.ParameterSets.ContainsKey($PSCmdlet.ParameterSetName)) {
            continue
        }
        $PSBoundParameters[$param.Key] = $ExecutionContext.SessionState.PSVariable.GetValue($param.Key)
    }

    $PSBoundParameters
} -ParamBoth foo

method3.ps1

$null = [psmoduleinfo]::new{
    Update-TypeData -MemberType ScriptMethod -MemberName SetParameterDefaultValues -Value {
        $boundParams = $this.MyInvocation.BoundParameters
        $enum = [System.Management.Automation.CommandMetadata]::new($this.MyInvocation.MyCommand).
            Parameters.GetEnumerator()

        foreach ($param in $enum) {
            if ($boundParams.ContainsKey($param.Key)) {
                continue
            }
            if (-not $param.Value.ParameterSets.ContainsKey($this.ParameterSetName)) {
                continue
            }
            $boundParams[$param.Key] = $this.GetVariableValue($param.Key)
        }
    } -Force -TypeName ([System.Management.Automation.PSCmdlet])
}

& {
    [CmdletBinding(DefaultParameterSetName = 'A')]
    param(
        [Parameter(ParameterSetName = 'A')]
        $ParamA = 'A',
        [Parameter(ParameterSetName = 'B')]
        $ParamB = 'B',
        [Parameter(Mandatory)]
        $ParamBoth
    )

    $PSCmdlet.SetParameterDefaultValues()
    $PSBoundParameters
} -ParamBoth foo