Fix handling of conflicting dynamic parameters

src/functions/Mock.ps1

@@ -1339,21 +1339,20 @@ function Get-ContextToDefine {
         }
         else {
             # the parameter is not defined in the parameter set,
-            # it is probably dynamic, let's see if I can get away with just adding
-            # it to the list of stuff to define
+            # it is probably dynamic, try remove "_" since the conflicting names
+            # are already handled to properly print the debug message
 
-            $name = if ($param.Key -in $script:ConflictingParameterNames) {
-                if ($PesterPreference.Debug.WriteDebugMessages.Value) {
-                    Write-PesterDebugMessage -Scope Mock -Message "! Variable `$$($param.Key) is a built-in variable, rewriting it to `$_$($param.Key). Use the version with _ in your -ParameterFilter."
+            if ($param.Key.StartsWith('_')) {
+                $originalName = $param.Key.TrimStart('_')
+                if ($originalName -in $script:ConflictingParameterNames) {
+                    if ($PesterPreference.Debug.WriteDebugMessages.Value) {
+                        Write-PesterDebugMessage -Scope Mock -Message "! Variable `$$($originalName) is a built-in variable, rewriting it to `$_$($originalName). Use the version with _ in your -ParameterFilter."
+                    }
                 }
-                "_$($param.Key)"
-            }
-            else {
-                $param.Key
             }
 
-            if (-not $r.ContainsKey($name)) {
-                $r.Add($name, $param.Value)
+            if (-not $r.ContainsKey($param.Key)) {
+                $r.Add($param.Key, $param.Value)
             }
         }
     }
@@ -1457,13 +1456,50 @@ function Get-MockDynamicParameter {
 
     switch ($PSCmdlet.ParameterSetName) {
         'Cmdlet' {
-            Get-DynamicParametersForCmdlet -CmdletName $CmdletName -Parameters $Parameters
+            $dynamicParams = Get-DynamicParametersForCmdlet -CmdletName $CmdletName -Parameters $Parameters
         }
 
         'Function' {
-            Get-DynamicParametersForMockedFunction -DynamicParamScriptBlock $DynamicParamScriptBlock -Parameters $Parameters -Cmdlet $Cmdlet
+            $dynamicParams = Get-DynamicParametersForMockedFunction -DynamicParamScriptBlock $DynamicParamScriptBlock -Parameters $Parameters -Cmdlet $Cmdlet
+        }
+    }
+
+    if ($null -eq $dynamicParams) {
+        return
+    }
+
+    Repair-ConflictingDynamicParameters -DynamicParams $dynamicParams
+}
+
+function Repair-ConflictingDynamicParameters {
+    [OutputType([System.Management.Automation.RuntimeDefinedParameterDictionary])]
+    param (
+        [Parameter(Mandatory = $true)]
+        [System.Management.Automation.RuntimeDefinedParameterDictionary]
+        $DynamicParams
+    )
+
+    $repairedDynamicParams = New-Object System.Management.Automation.RuntimeDefinedParameterDictionary
+    $conflictingParams = Get-ConflictingParameterNames
+
+    foreach ($paramName in $DynamicParams.Keys) {
+        $dynamicParam = $DynamicParams[$paramName]
+
+        if ($conflictingParams -contains $paramName) {
+            $newName = "_$paramName"
+            $dynamicParam.Name = $newName
+
+            $aliasAttribute = New-Object System.Management.Automation.AliasAttribute -ArgumentList $paramName
+            $dynamicParam.Attributes.Add($aliasAttribute)
+
+            $repairedDynamicParams[$newName] = $dynamicParam
+        }
+        else {
+            $repairedDynamicParams[$paramName] = $dynamicParam
         }
     }
+
+    return $repairedDynamicParams
 }
 
 function Get-DynamicParametersForCmdlet {

tst/functions/Mock.Tests.ps1

@@ -3160,3 +3160,72 @@ Describe 'Mocking with nested Pester runs' {
         Get-Command Get-ChildItem | Should -Not -Be 2
     }
 }
+
+Describe 'Usage of Alias in DynamicParams' {
+    # https://github.com/pester/Pester/issues/1274
+
+    BeforeAll {
+        function New-DynamicAttr($ParamDictionary, $Name, $Alias = $null) {
+            $attr = New-Object -Type `
+                System.Management.Automation.ParameterAttribute
+            $attr.Mandatory = $false
+            $attr.ParameterSetName = '__AllParameterSets'
+            $attributeCollection = New-Object `
+                -Type System.Collections.ObjectModel.Collection[System.Attribute]
+            $attributeCollection.Add($attr)
+
+            if ($null -ne $Alias) {
+                $attr = New-Object -Type `
+                    System.Management.Automation.AliasAttribute -ArgumentList @($Alias)
+                $attributeCollection.Add($attr)
+            }
+
+            $dynParam1 = New-Object -Type `
+                System.Management.Automation.RuntimeDefinedParameter($Name, [string],
+                $attributeCollection)
+
+            $ParamDictionary.Add($Name, $dynParam1)
+        }
+
+        function Test-DynamicParam {
+            [CmdletBinding()]
+            param(
+                [String]$Name
+            )
+
+            dynamicparam {
+                if ($Name.StartsWith("Hello")) {
+                    $paramDictionary = New-Object `
+                        -Type System.Management.Automation.RuntimeDefinedParameterDictionary
+                    New-DynamicAttr -ParamDictionary $paramDictionary -Name "PSEdition"
+
+                    return $paramDictionary
+                }
+            }
+
+            process {
+                if ($PSBoundParameters.PSEdition) {
+                    Write-Host "PSEdition value: $($PSBoundParameters.PSEdition)"
+                }
+            }
+        }
+    }
+
+    Context 'Mocking with ParameterFilter' {
+        It 'Mocks Test-DynamicParam with PSEdition set to Desktop' {
+            Mock Test-DynamicParam { "World" } -ParameterFilter { $_PSEdition -eq 'Desktop' }
+
+            Test-DynamicParam -Name "Hello" -PSEdition 'Desktop' | Should -Be 'World'
+        }
+    }
+
+    Context 'Validating Mock Invocation' {
+        It 'Invokes Test-DynamicParam with correct parameters' {
+            Mock Test-DynamicParam { "World" }
+
+            Test-DynamicParam -Name "Hello" -PSEdition 'Desktop' | Should -Be 'World'
+
+            Should -Invoke Test-DynamicParam -Exactly 1 -Scope It
+        }
+    }
+}