Add tests for custom assertions (#2552)

* Add tests for custom assertions

* Fix test for strict mode

tst/functions/Add-ShouldOperator.ts.ps1

@@ -70,6 +70,78 @@ i -PassThru:$PassThru {
         }
     }
 
+    b 'Executing custom Should assertions' {
+        # Testing paramter and output syntax described in docs (https://pester.dev/docs/assertions/custom-assertions)
+        Get-Module Pester | Remove-Module
+        Import-Module "$PSScriptRoot\..\..\bin\Pester.psd1"
+
+        t 'Works for simple functions with expected output' {
+            function CustomSimple ([string] $ActualValue, [switch] $Negate) {
+                $succeeded = 'A' -eq $ActualValue
+                if ($Negate) { $succeeded = -not $succeeded }
+
+                # Default failure message. Not used by Should unless succeeded is $false
+                $failureMessage = "Expected 'A', but got '$ActualValue'."
+                if (-not $succeeded -and $Negate) {
+                    $failureMessage = "Expected anything but 'A', but got '$ActualValue'."
+                }
+
+                return [pscustomobject]@{
+                    Succeeded      = $succeeded
+                    FailureMessage = $failureMessage
+                }
+            }
+
+            Add-ShouldOperator -Name CustomSimple -Test $function:CustomSimple
+            'A' | Should -CustomSimple
+            'B' | Should -CustomSimple -Not
+            $e = { 'B' | Should -CustomSimple } | Verify-AssertionFailed
+            $e.Exception.Message | Should -Be "Expected 'A', but got 'B'."
+        }
+
+        t 'Works for advanced function with expected output' {
+            function CustomAdvanced {
+                [CmdletBinding()]
+                # Requires all mandatory parameters below as $args is disabled by default in advanced functions
+                param(
+                    [string] $ActualValue,
+                    [switch] $Negate,
+                    $CallerSessionState
+                )
+                $succeeded = 'A' -eq $ActualValue
+                if ($Negate) { $succeeded = -not $succeeded }
+
+                # Default failure message. Not used by Should unless succeeded is $false
+                $failureMessage = "Expected 'A', but got '$ActualValue'."
+                if (-not $succeeded -and $Negate) {
+                    $failureMessage = "Expected anything but 'A', but got '$ActualValue'."
+                }
+
+                return [pscustomobject]@{
+                    Succeeded      = $succeeded
+                    FailureMessage = $failureMessage
+                }
+            }
+
+            Add-ShouldOperator -Name CustomAdvanced -Test $function:CustomAdvanced
+            'A' | Should -CustomAdvanced
+            'B' | Should -CustomAdvanced -Not
+            $e = { 'B' | Should -CustomAdvanced } | Verify-AssertionFailed
+            $e.Exception.Message | Should -Be "Expected 'A', but got 'B'."
+        }
+
+        t 'Assertion exceptions are surfaced to user' {
+            function CustomException ([string] $ActualValue, [switch] $Negate) {
+                throw [System.InvalidOperationException]'something went wrong'
+            }
+
+            Add-ShouldOperator -Name CustomException -Test $function:CustomException
+            $e = { 'A' | Should -CustomException } | Verify-Throw
+            $e.Exception | Verify-Type -Expected ([System.InvalidOperationException])
+            $e.Exception.Message | Should -Be "something went wrong"
+        }
+    }
+
     b 'HelpMessage for built-in Should operators' {
         Get-Module Pester | Remove-Module
         Import-Module "$PSScriptRoot\..\..\bin\Pester.psd1"