Merge pull request #24069 from mavasani/DiagnosticCreateCrash

Fix ArgumentException when creating diagnostic with effective severit…

src/Compilers/Core/CodeAnalysisTest/Diagnostics/DiagnosticCreationTests.cs

+// Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
+
+using Roslyn.Test.Utilities;
+using Xunit;
+
+namespace Microsoft.CodeAnalysis.UnitTests.Diagnostics
+{
+    public partial class DiagnosticCreationTests
+    {
+        [Fact, WorkItem(547049, "https://devdiv.visualstudio.com/DevDiv/_workitems?id=547049")]
+        public void TestDiagnosticCreationWithOverriddenSeverity()
+        {
+            var defaultSeverity = DiagnosticSeverity.Info;
+            var effectiveSeverity = DiagnosticSeverity.Error;
+            var descriptor = new DiagnosticDescriptor("ID", "Title", "MessageFormat", "Category", defaultSeverity, isEnabledByDefault: true);
+            var diagnostic = Diagnostic.Create(descriptor, Location.None, effectiveSeverity, additionalLocations: null, properties: null);
+            Assert.Equal(effectiveSeverity, diagnostic.Severity);
+            Assert.Equal(0, diagnostic.WarningLevel);
+        }
+    }
+}

src/Compilers/Core/Portable/Diagnostic/Diagnostic.cs

@@ -139,7 +139,7 @@ public abstract partial class Diagnostic : IEquatable<Diagnostic>, IFormattable
                 throw new ArgumentNullException(nameof(descriptor));
             }
 
-            var warningLevel = GetDefaultWarningLevel(descriptor.DefaultSeverity);
+            var warningLevel = GetDefaultWarningLevel(effectiveSeverity);
             return SimpleDiagnostic.Create(
                 descriptor,
                 severity: effectiveSeverity,