Implement ?? operator

Author: adityapatwardhan

src/System.Management.Automation/engine/parser/Compiler.cs

@@ -5247,6 +5247,15 @@ public object VisitBinaryExpression(BinaryExpressionAst binaryExpressionAst)
                         CachedReflectionInfo.ParserOps_SplitOperator,
                         _executionContextParameter, Expression.Constant(binaryExpressionAst.ErrorPosition), lhs.Cast(typeof(object)), rhs.Cast(typeof(object)),
                         ExpressionCache.Constant(false));
+                case TokenKind.QuestionQuestion:
+                    if (lhs is ConstantExpression lhsConstExpr && lhsConstExpr.Value != null)
+                    {
+                        return lhs;
+                    }
+                    else
+                    {
+                        return Expression.Coalesce(lhs, rhs);
+                    }
             }
 
             throw new InvalidOperationException("Unknown token in binary operator.");

src/System.Management.Automation/engine/parser/token.cs

@@ -419,6 +419,9 @@ public enum TokenKind
         /// <summary>The null conditional assignment operator '?='.</summary>
         QuestionEquals = 101,
 
+        /// <summary>The null conditional assignment operator '?='.</summary>
+        QuestionQuestion = 101,
+
         #endregion Operators
 
         #region Keywords
@@ -858,7 +861,7 @@ public static class TokenTraits
             /*                Colon */ TokenFlags.SpecialOperator | TokenFlags.DisallowedInRestrictedMode,
             /*         QuestionMark */ TokenFlags.TernaryOperator | TokenFlags.DisallowedInRestrictedMode,
             /*      QuestionEquals  */ TokenFlags.AssignmentOperator,
-            /*     Reserved slot 4  */ TokenFlags.None,
+            /*    QuestionQuestion  */ TokenFlags.BinaryOperator,
             /*     Reserved slot 5  */ TokenFlags.None,
             /*     Reserved slot 6  */ TokenFlags.None,
             /*     Reserved slot 7  */ TokenFlags.None,

src/System.Management.Automation/engine/parser/tokenizer.cs

@@ -5005,6 +5005,12 @@ internal Token NextToken()
                         return CheckOperatorInCommandMode(c, c1, TokenKind.QuestionEquals);
                     }
 
+                    if (c1 == '?')
+                    {
+                        SkipChar();
+                        return CheckOperatorInCommandMode(c, c1, TokenKind.QuestionQuestion);
+                    }
+
                     return ScanGenericToken(c);
 
                 case '\0':

test/powershell/Language/Operators/NullConditional.Tests.ps1

@@ -0,0 +1,110 @@
+# Copyright (c) Microsoft Corporation. All rights reserved.
+# Licensed under the MIT License.
+
+Describe 'NullConditionalOperations' -tag 'CI' {
+
+    Context "Null conditional assignment operator ?=" {
+        BeforeAll {
+            $someGuid = New-Guid
+
+            $typesTests = @(
+                @{ name = 'string'; valueToSet = 'hello' }
+                @{ name = 'dotnetType'; valueToSet = $someGuid }
+                @{ name = 'byte'; valueToSet = [byte]0x94 }
+                @{ name = 'intArray'; valueToSet = 1..2 }
+                @{ name = 'stringArray'; valueToSet = 'a'..'c' }
+                @{ name = 'emptyArray'; valueToSet = @(1, 2, 3) }
+            )
+
+        }
+
+        It 'Variable doesnot exist' {
+            Remove-Variable variableDoesNotExist -ErrorAction SilentlyContinue -Force
+
+            $variableDoesNotExist ?= 1
+            $variableDoesNotExist | Should -Be 1
+
+            $variableDoesNotExist ?= 2
+            $variableDoesNotExist | Should -Be 1
+        }
+
+        It 'Variable exists and is null' {
+            $variableDoesNotExist = $null
+
+            $variableDoesNotExist ?= 2
+            $variableDoesNotExist | Should -Be 2
+        }
+
+        It 'Validate types - <name> can be set' -TestCases $typesTests {
+            param ($name, $valueToSet)
+
+            $x = $null
+            $x ?= $valueToSet
+            $x | Should -Be $valueToSet
+        }
+
+        It 'Validate hashtable can be set' {
+            $x = $null
+            $x ?= @{ 1 = '1' }
+            $x.Keys | Should -Be @(1)
+        }
+
+        It 'Validate lhs is returned' {
+            $x = 100
+            $x ?= 200
+            $x | Should -Be 100
+        }
+
+        It 'Error case' {
+            $e = $null
+            $null = [System.Management.Automation.Language.Parser]::ParseInput('1 ?= 100', [ref] $null, [ref] $e)
+            $e[0].ErrorId | Should -BeExactly 'InvalidLeftHandSide'
+        }
+    }
+
+    Context 'Null coalesce operator ??' {
+
+        It 'Variable does not exist' {
+            $variableDoesNotExist ?? 100 | Should -Be 100
+        }
+
+        It 'Variable exists but is null' {
+            $x = $null
+            $x ?? 100 | Should -Be 100
+        }
+
+        It 'Lhs is not null' {
+            $x = 100
+            $x ?? 200 | Should -Be 100
+        }
+
+        It 'Lhs is a non-null constant' {
+            1 ?? 2 | Should -Be 1
+        }
+
+        It 'Lhs is `$null' {
+            $null ?? 'string value' | Should -BeExactly 'string value'
+        }
+
+        It 'Check precedence of ?? expression resolution' {
+            $x ?? $null ?? 100 | Should -Be 100
+            $null ?? $null ?? 100 | Should -Be 100
+            $null ?? $null ?? $null | Should -Be $null
+            $x ?? 200 ?? $null | Should -Be 200
+            $x ?? 200 ?? 300 | Should -Be 200
+            100 ?? $x ?? 200 | Should -Be 100
+            $null ?? 100 ?? $null ?? 200 | Should -Be 100
+        }
+    }
+
+    Context 'Combined usage of null conditional operators' {
+
+        It '?? and ?= used together' {
+            $x ?= 100 ?? 200
+            $x | Should -Be 100
+
+            $y ?= 100 ?? 200
+            $y | Should -Be 100
+        }
+    }
+}

test/powershell/Language/Operators/NullCondtional.Tests.ps1

@@ -260,6 +260,12 @@ Describe 'function statement parsing' -Tags "CI" {
 
 Describe 'assignment statement parsing' -Tags "CI" {
     ShouldBeParseError '$a,$b += 1,2' InvalidLeftHandSide 0
+    ShouldBeParseError '1 ?= 1' InvalidLeftHandSide 0
+    ShouldBeParseError '@() ?= 1' InvalidLeftHandSide 0
+    ShouldBeParseError '@{} ?= 1' InvalidLeftHandSide 0
+    ShouldBeParseError '1..2 ?= 1' InvalidLeftHandSide 0
+    ShouldBeParseError '[int] ?= 1' InvalidLeftHandSide 0
+    ShouldBeParseError '(Get-Variable x) ?= 1' InvalidLeftHandSide 0
 }
 
 Describe 'splatting parsing' -Tags "CI" {