Fix #2953: fail open when a bound parameter's ToString throws in the mock filter serializer

src/functions/Mock.ps1

@@ -1259,6 +1259,22 @@ function Invoke-InMockScope {
     }
 }
 
+function Format-BoundParameterValueSafely {
+    # Stringify a bound-parameter value for diagnostic text (debug messages and the failed-filter
+    # summary). This is only ever used to provide context to the user, so it must fail open: some
+    # types have a ToString that throws (e.g. mocked SMO objects created via New-MockObject), and
+    # a value like that must not make the whole mock filter throw when it is not even referenced by
+    # the filter. See #2953.
+    param($Value)
+
+    try {
+        "$Value"
+    }
+    catch {
+        '<value could not be serialized>'
+    }
+}
+
 function Test-ParameterFilter {
     [CmdletBinding()]
     param (
@@ -1319,7 +1335,7 @@ function Test-ParameterFilter {
 
     if ($PesterPreference.Debug.WriteDebugMessages.Value) {
         $hasContext = 0 -lt $Context.Count
-        $c = $(if ($hasContext) { foreach ($p in $Context.GetEnumerator()) { "$($p.Key) = $($p.Value)" } }) -join ", "
+        $c = $(if ($hasContext) { foreach ($p in $Context.GetEnumerator()) { "$($p.Key) = $(Format-BoundParameterValueSafely $p.Value)" } }) -join ", "
         Write-PesterDebugMessage -Scope Mock -Message "Running mock filter { $scriptBlock } $(if ($hasContext) { "with context: $c" } else { "without any context"})."
     }
 
@@ -1367,7 +1383,7 @@ function Test-ParameterFilter {
         $filterText = $scriptBlock.ToString().Trim()
         $hasContext = 0 -lt $Context.Count
         $contextText = if ($hasContext) {
-            'bound parameters: ' + (($Context.GetEnumerator() | & $SafeCommands['ForEach-Object'] { "$($_.Key) = $($_.Value)" }) -join ', ')
+            'bound parameters: ' + (($Context.GetEnumerator() | & $SafeCommands['ForEach-Object'] { "$($_.Key) = $(Format-BoundParameterValueSafely $_.Value)" }) -join ', ')
         }
         else {
             'no bound parameters'

tst/functions/Mock.Tests.ps1

@@ -337,6 +337,30 @@ Describe 'When calling Mock, StrictMode is enabled, and variables are used in th
     }
 }
 
+Describe 'When a bound parameter value has a ToString that throws' {
+    # The parameter filter serializer only builds diagnostic text, so a value whose ToString throws
+    # (e.g. a mocked SMO type) must not make the mock throw when the value is not even referenced by
+    # the filter. See #2953.
+    BeforeAll {
+        function Get-Thing {
+            param (
+                [object] $InputObject,
+                [switch] $Other
+            )
+        }
+    }
+
+    It 'Does not throw when a non-matching parameter filter is present' {
+        $throwingToString = [pscustomobject]@{ Name = 'demo' }
+        $throwingToString | Add-Member -MemberType ScriptMethod -Name ToString -Value { throw 'ToString should not be called by the parameter filter serializer' } -Force
+
+        Mock Get-Thing { 'default' }
+        Mock Get-Thing -ParameterFilter { $Other.IsPresent } { 'other' }
+
+        { Get-Thing -InputObject $throwingToString } | Should -Not -Throw
+    }
+}
+
 Describe "When calling Mock on existing function without matching bound params" {
     It "Should throw because no parameter filter matched the call" {
         Mock FunctionUnderTest { return "fake results" } -parameterFilter { $param1 -eq "test" }