Do not suggest or diagnose use compound assignment when right hand of…

src/EditorFeatures/CSharpTest/UseCompoundAssignment/UseCompoundAssignmentTests.cs

@@ -251,6 +251,23 @@ void M(int? a)
     new TestParameters(parseOptions: new CSharpParseOptions(LanguageVersion.CSharp7_3)));
         }
 
+        [Fact]
+        [WorkItem(36467, "https://github.com/dotnet/roslyn/issues/36467")]
+        [Trait(Traits.Feature, Traits.Features.CodeActionsUseCompoundAssignment)]
+        public async Task TestNotSuggestedWhenRightHandIsThrowExpression()
+        {
+            await TestMissingAsync(
+@"using System;
+public class C
+{
+    void M(int? a)
+    {
+        a [||]= a ?? throw new Exception();
+    }
+}",
+    new TestParameters(parseOptions: new CSharpParseOptions(LanguageVersion.CSharp8)));
+        }
+
         [Fact, Trait(Traits.Feature, Traits.Features.CodeActionsUseCompoundAssignment)]
         public async Task TestField()
         {

src/Features/Core/Portable/UseCompoundAssignment/AbstractUseCompoundAssignmentDiagnosticAnalyzer.cs

@@ -94,7 +94,7 @@ private void AnalyzeAssignment(SyntaxNodeAnalysisContext context)
             }
 
             _syntaxFacts.GetPartsOfBinaryExpression(binaryExpression,
-                out var binaryLeft, out _);
+                out var binaryLeft, out var binaryRight);
 
             // has to be of the form:   expr = expr op ...
             if (!_syntaxFacts.AreEquivalent(assignmentLeft, binaryLeft))
@@ -109,6 +109,12 @@ private void AnalyzeAssignment(SyntaxNodeAnalysisContext context)
                 return;
             }
 
+            // Don't offer if this is `x = x ?? throw new Exception()`
+            if (_syntaxFacts.IsThrowExpression(binaryRight))
+            {
+                return;
+            }
+
             // Syntactically looks promising.  But we can only safely do this if 'expr'
             // is side-effect-free since we will be changing the number of times it is
             // executed from twice to once.