Don't offer to inline _ as that could become a discard.

src/EditorFeatures/CSharpTest/InlineDeclaration/CSharpInlineDeclarationTests.cs

@@ -1610,5 +1610,25 @@ void Foo()
     }
 }", ignoreTrivia: false);
         }
+
+        [Fact, Trait(Traits.Feature, Traits.Features.CodeActionsInlineDeclaration)]
+        public async Task TestMissingOnUnderscore()
+        {
+            await TestMissingInRegularAndScriptAsync(
+@"
+using System;
+
+class C
+{
+    void M()
+    {
+        [|int|] _;
+        if (N(out _)
+        {
+            Console.WriteLine(_);
+        }
+    }
+}");
+        }
     }
 }
\ No newline at end of file

src/Features/CSharp/Portable/InlineDeclaration/CSharpInlineDeclarationDiagnosticAnalyzer.cs

@@ -113,6 +113,13 @@ private void AnalyzeSyntaxNode(SyntaxNodeAnalysisContext context, INamedTypeSymb
 
             var identifierName = (IdentifierNameSyntax)argumentExpression;
 
+            // Don't offer to inline variables named "_".  It can cause is to create a discard symbol
+            // which would cause a break.
+            if (identifierName.Identifier.ValueText == "_")
+            {
+                return;
+            }
+
             var containingStatement = argumentExpression.FirstAncestorOrSelf<StatementSyntax>();
             if (containingStatement == null)
             {