Use explicit type from lambda expression (RCS1008)

ChangeLog.md

@@ -34,7 +34,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Fix formatting of argument list ([#952](https://github.com/josefpihrt/roslynator/pull/952).
 - Do not remove async/await when 'using declaration' is used ([#953](https://github.com/josefpihrt/roslynator/pull/953).
 - Convert if-else to return statement when pattern matching is used ([RCS1073](https://github.com/JosefPihrt/Roslynator/blob/main/docs/analyzers/RCS1073.md)) ([#956](https://github.com/josefpihrt/roslynator/pull/956).
-- Do not simplify 'default' expression if the type is inferred ([RCS1244](https://github.com/JosefPihrt/Roslynator/blob/main/docs/analyzers/1244.md)) ([#956](https://github.com/josefpihrt/roslynator/pull/966).
+- Do not simplify 'default' expression if the type is inferred ([RCS1244](https://github.com/JosefPihrt/Roslynator/blob/main/docs/analyzers/RCS1244.md)) ([#966](https://github.com/josefpihrt/roslynator/pull/966).
+- Use explicit type from lambda expression ([RCS1008](https://github.com/JosefPihrt/Roslynator/blob/main/docs/analyzers/RCS1008.md)) ([#967](https://github.com/josefpihrt/roslynator/pull/967).
 
 -----
 <!-- Content below does not adhere to 'Keep a Changelog' format -->

src/Analyzers.CodeFixes/CSharp/CodeFixes/UseExplicitTypeInsteadOfVarCodeFixProvider.cs

@@ -2,6 +2,7 @@
 
 using System.Collections.Immutable;
 using System.Composition;
+using System.Diagnostics;
 using System.Threading.Tasks;
 using Microsoft.CodeAnalysis;
 using Microsoft.CodeAnalysis.CodeActions;
@@ -34,24 +35,45 @@ public override async Task RegisterCodeFixesAsync(CodeFixContext context)
             if (!TryFindFirstAncestorOrSelf(root, context.Span, out SyntaxNode node, predicate: f => f.IsKind(SyntaxKind.VariableDeclaration, SyntaxKind.DeclarationExpression)))
                 return;
 
+            SemanticModel semanticModel = await context.GetSemanticModelAsync().ConfigureAwait(false);
+
             if (node is VariableDeclarationSyntax variableDeclaration)
             {
-                TypeSyntax type = variableDeclaration.Type;
+                ExpressionSyntax value = variableDeclaration.Variables[0].Initializer.Value;
+                ITypeSymbol typeSymbol = semanticModel.GetTypeSymbol(value, context.CancellationToken);
 
-                SemanticModel semanticModel = await context.GetSemanticModelAsync().ConfigureAwait(false);
+                if (typeSymbol is null)
+                {
+                    var localSymbol = semanticModel.GetDeclaredSymbol(variableDeclaration.Variables[0], context.CancellationToken) as ILocalSymbol;
 
-                ITypeSymbol typeSymbol = semanticModel.GetTypeSymbol(variableDeclaration.Variables[0].Initializer.Value, context.CancellationToken);
+                    if (localSymbol is not null)
+                    {
+                        typeSymbol = localSymbol.Type;
 
-                RegisterCodeFix(context, type, typeSymbol, semanticModel);
+                        value = value.WalkDownParentheses();
+
+                        Debug.Assert(
+                            value.IsKind(SyntaxKind.SimpleLambdaExpression, SyntaxKind.ParenthesizedLambdaExpression),
+                            value.Kind().ToString());
+
+                        if (value.IsKind(SyntaxKind.SimpleLambdaExpression, SyntaxKind.ParenthesizedLambdaExpression))
+                            typeSymbol = typeSymbol.WithNullableAnnotation(NullableAnnotation.NotAnnotated);
+                    }
+                    else
+                    {
+                        SyntaxDebug.Fail(variableDeclaration.Variables[0]);
+                        return;
+                    }
+                }
+
+                RegisterCodeFix(context, variableDeclaration.Type, typeSymbol, semanticModel);
             }
             else
             {
                 var declarationExpression = (DeclarationExpressionSyntax)node;
 
                 TypeSyntax type = declarationExpression.Type;
 
-                SemanticModel semanticModel = await context.GetSemanticModelAsync().ConfigureAwait(false);
-
                 var localSymbol = semanticModel.GetDeclaredSymbol(declarationExpression.Designation, context.CancellationToken) as ILocalSymbol;
 
                 ITypeSymbol typeSymbol = (localSymbol?.Type) ?? semanticModel.GetTypeSymbol(declarationExpression, context.CancellationToken);

src/Tests/Analyzers.Tests/RCS1008UseExplicitTypeInsteadOfVarWhenTypeIsNotObviousTests.cs

@@ -249,6 +249,66 @@ void M()
 ");
         }
 
+        [Fact, Trait(Traits.Analyzer, DiagnosticIdentifiers.UseExplicitTypeInsteadOfVarWhenTypeIsNotObvious)]
+        public async Task Test_Func_Lambda()
+        {
+            await VerifyDiagnosticAndFixAsync(@"
+class C
+{
+    void M()
+    {
+        [|var|] x = () =>
+        {
+            return default(object);
+        };
+    }
+}
+", @"
+class C
+{
+    void M()
+    {
+        System.Func<object> x = () =>
+        {
+            return default(object);
+        };
+    }
+}
+", options: Options.AddAllowedCompilerDiagnosticId("CS8603"));
+        }
+
+        [Fact, Trait(Traits.Analyzer, DiagnosticIdentifiers.UseExplicitTypeInsteadOfVarWhenTypeIsNotObvious)]
+        public async Task Test_Func_Lambda_Nullable()
+        {
+            await VerifyDiagnosticAndFixAsync(@"
+#nullable enable
+
+class C
+{
+    void M()
+    {
+        [|var|] x = () =>
+        {
+            return default(object);
+        };
+    }
+}
+", @"
+#nullable enable
+
+class C
+{
+    void M()
+    {
+        System.Func<object> x = () =>
+        {
+            return default(object);
+        };
+    }
+}
+", options: Options.AddAllowedCompilerDiagnosticId("CS8603"));
+        }
+
         [Fact, Trait(Traits.Analyzer, DiagnosticIdentifiers.UseExplicitTypeInsteadOfVarWhenTypeIsNotObvious)]
         public async Task TestNoDiagnostic()
         {