Bail out on error type in CSharpTypeStyleDiagnosticAnalyzer

Fixes #29718

src/EditorFeatures/CSharpTest/Diagnostics/UseImplicitOrExplicitType/UseExplicitTypeTests.cs

@@ -235,6 +235,27 @@ void Method()
 }", new TestParameters(options: ExplicitTypeEverywhere()));
         }
 
+        [WorkItem(29718, "https://github.com/dotnet/roslyn/issues/29718")]
+        [WpfFact, Trait(Traits.Feature, Traits.Features.CodeActionsUseExplicitType)]
+        public async Task NotOnErrorConvertedType()
+        {
+            await TestMissingInRegularAndScriptAsync(
+ @"using System;
+using System.Collections.Generic;
+
+class C
+{
+    void M()
+    {
+        // Error CS1061: 'KeyValuePair<int, int>' does not contain a definition for 'Deconstruct' and no accessible extension method 'Deconstruct' accepting a first argument of type 'KeyValuePair<int, int>' could be found (are you missing a using directive or an assembly reference?)
+        // Error CS8129:	No suitable 'Deconstruct' instance or extension method was found for type 'KeyValuePair<int, int>', with 2 out parameters and a void return type.
+        foreach ([|var|] (key, value) in new Dictionary<int, int>())
+        {
+        }
+    }
+}", new TestParameters(options: ExplicitTypeEverywhere()));
+        }
+
         #endregion
 
         [WpfFact, Trait(Traits.Feature, Traits.Features.CodeActionsUseExplicitType)]

src/Features/CSharp/Portable/Diagnostics/Analyzers/CSharpTypeStyleDiagnosticAnalyzerBase.cs

@@ -5,6 +5,7 @@
 using Microsoft.CodeAnalysis.CSharp.Extensions;
 using Microsoft.CodeAnalysis.CSharp.Utilities;
 using Microsoft.CodeAnalysis.Diagnostics;
+using Microsoft.CodeAnalysis.Shared.Extensions;
 using Microsoft.CodeAnalysis.Text;
 
 namespace Microsoft.CodeAnalysis.CSharp.Diagnostics.TypeStyle
@@ -56,6 +57,15 @@ private void HandleVariableDeclaration(SyntaxNodeAnalysisContext context)
                 return;
             }
 
+            var typeInfo = semanticModel.GetTypeInfo(declaredType, cancellationToken);
+            if (typeInfo.Type == null ||
+                typeInfo.Type.IsErrorType() ||
+                typeInfo.ConvertedType == null ||
+                typeInfo.ConvertedType.IsErrorType())
+            {
+                return;
+            }
+
             var typeStyle = Helper.AnalyzeTypeName(
                 declaredType, semanticModel, optionSet, cancellationToken);
             if (!typeStyle.IsStylePreferred || !typeStyle.CanConvert())