Check that every test input clones without changes

AGENTS.md

@@ -160,6 +160,10 @@ come back empty.
 - Tests compile against real framework assemblies via `TestHelper`
 - `TestHelper` fails a test whose generated code does not compile, before any
   snapshot is compared, and reports the compiler errors
+- `TestHelper` also clones every synchronized method through
+  `IdentityCloneGenerator`, which changes nothing but the method's name, and
+  fails the test with "Identity clone does not compile" when a clone does not.
+  That failure is in the `Cloning/` layer, not in the async to sync rules
 
 ### Choosing how the source is wrapped
 

src/Zomp.SyncMethodGenerator/Zomp.SyncMethodGenerator.csproj

@@ -54,6 +54,11 @@
     <Using Include="Zomp.SyncMethodGenerator.DiagnosticMessages" Static="true" />
   </ItemGroup>
 
+  <ItemGroup>
+    <!-- The tests clone methods through the Cloning layer directly -->
+    <InternalsVisibleTo Include="Generator.Tests" />
+  </ItemGroup>
+
   <ItemGroup>
     <PackageReference Update="Microsoft.SourceLink.GitHub" PrivateAssets="all" IncludeAssets="runtime; build; native; contentfiles; analyzers" />
   </ItemGroup>

tests/Generator.Tests/IdentityCloneGenerator.cs

@@ -0,0 +1,68 @@
+using Zomp.SyncMethodGenerator;
+using Zomp.SyncMethodGenerator.Cloning;
+
+namespace Generator.Tests;
+
+/// <summary>
+/// Clones every method the sync generator would synchronize, through <see cref="IdentityRewriter"/>,
+/// which changes nothing but its name. A clone which does not compile points at the cloning layer
+/// rather than at the async to sync rules.
+/// </summary>
+internal sealed class IdentityCloneGenerator : IIncrementalGenerator
+{
+    /// <inheritdoc/>
+    public void Initialize(IncrementalGeneratorInitializationContext context)
+    {
+        var disableNullable = context.CompilationProvider.Select(static (c, _) =>
+            c.Options.NullableContextOptions == NullableContextOptions.Disable
+            || c is CSharpCompilation { LanguageVersion: < LanguageVersion.CSharp8 });
+
+        var methods = CloneTarget.ForAttribute(context.SyntaxProvider, SyncMethodSourceGenerator.QualifiedCreateSyncVersionAttribute)
+            .Combine(disableNullable)
+            .Select(static (data, ct) => Clone(data.Left, data.Right, ct)!)
+            .Where(static m => m is not null);
+
+        ClonedMethodOutput.Register(context, methods, Zomp.SyncMethodGenerator.DiagnosticMessages.CollidingOverloads);
+    }
+
+    private static ClonedMethod? Clone(CloneTarget target, bool disableNullable, CancellationToken ct)
+    {
+        var context = target.Context;
+        var isTargetTypeSymbol = context.TargetSymbol is ITypeSymbol;
+
+        var symbol = isTargetTypeSymbol
+            ? context.SemanticModel.GetDeclaredSymbol(target.Syntax, ct)
+            : context.TargetSymbol as IMethodSymbol;
+
+        // The same selection the sync generator makes, so that only the methods it copies are cloned.
+        if (symbol is null
+            || (!symbol.IsAsync
+                && (symbol.ReturnType is not INamedTypeSymbol named || !AsyncToSyncRewriter.IsTypeOfInterest(named))))
+        {
+            return null;
+        }
+
+        foreach (var attribute in symbol.GetAttributes())
+        {
+            var name = attribute.AttributeClass?.ToDisplayString();
+
+            if (name == SyncMethodSourceGenerator.QualifiedSkipSyncVersionAttribute
+                || (isTargetTypeSymbol && name == SyncMethodSourceGenerator.QualifiedCreateSyncVersionAttribute))
+            {
+                return null;
+            }
+        }
+
+        if (!MethodLocation.TryCreate(target.Syntax, symbol, out var location, out var root))
+        {
+            return null;
+        }
+
+        disableNullable |= context.Attributes[0].NamedArguments
+            .Any(static a => a is { Key: SyncMethodSourceGenerator.OmitNullableDirective, Value.Value: true });
+
+        var rewritten = new IdentityRewriter(context.SemanticModel, target.Syntax).Visit(root);
+
+        return ClonedMethod.Create(location, target.Syntax, rewritten, disableNullable, []);
+    }
+}

tests/Generator.Tests/IdentityRewriter.cs

@@ -0,0 +1,35 @@
+using Microsoft.CodeAnalysis.CSharp.Syntax;
+using Zomp.SyncMethodGenerator.Cloning;
+
+namespace Generator.Tests;
+
+/// <summary>
+/// A transformation which changes nothing but the name of the method, leaving everything else
+/// to <see cref="CloningRewriter"/>.
+/// </summary>
+/// <param name="semanticModel">The semantic model.</param>
+/// <param name="targetMethod">The method declaration to clone.</param>
+internal sealed class IdentityRewriter(SemanticModel semanticModel, MethodDeclarationSyntax targetMethod)
+    : CloningRewriter(semanticModel, targetMethod)
+{
+    /// <summary>
+    /// Appended to the name of each method, so that the clone does not collide with the original.
+    /// </summary>
+    internal const string Suffix = "Clone";
+
+    /// <inheritdoc/>
+    public override SyntaxNode? VisitMethodDeclaration(MethodDeclarationSyntax node)
+    {
+        if (base.VisitMethodDeclaration(node) is not MethodDeclarationSyntax clone)
+        {
+            return null;
+        }
+
+        var name = SyntaxFactory.Identifier(node.Identifier.ValueText + Suffix).WithTriviaFrom(clone.Identifier);
+
+        // Directives leading the method belong to the file it was written in.
+        return clone
+            .WithIdentifier(name)
+            .WithLeadingTrivia(RemovePreprocessorDirectives(clone.GetLeadingTrivia()));
+    }
+}

tests/Generator.Tests/TestHelper.cs

@@ -134,6 +134,8 @@ namespace Test;
                 "Compilation errors:\n" + string.Join("\n", diagnostics.Select(d => d.ToString())));
         }
 
+        EnsureIdentityClonesCompile(compilation, parseOptions);
+
         var target = new RunResultWithIgnoreList
         {
             Result = driver.GetRunResult(),
@@ -177,6 +179,51 @@ internal static string ChangeIndentation(string source, Func<string, string> fun
         return linesWithIndentation;
     }
 
+    /// <summary>
+    /// Clones every method the test synchronizes, changing nothing but its name, and fails the
+    /// test when a clone does not compile. Such a failure is in the cloning layer, whatever the
+    /// test itself is about.
+    /// </summary>
+    private static void EnsureIdentityClonesCompile(CSharpCompilation compilation, CSharpParseOptions parseOptions)
+    {
+        // The sync generator runs as well, since it declares the attributes the source uses.
+        var result = CSharpGeneratorDriver.Create(new SyncMethodSourceGenerator(), new IdentityCloneGenerator())
+            .WithUpdatedParseOptions(parseOptions)
+            .RunGenerators(compilation)
+            .GetRunResult();
+
+        var identity = result.Results.Single(r => r.Generator.GetGeneratorType() == typeof(IdentityCloneGenerator));
+
+        if (identity.Exception is { } exception)
+        {
+            throw new InvalidOperationException("Identity clone threw", exception);
+        }
+
+        // Every method which is synchronized is cloned as well, so that a pass which cloned
+        // nothing cannot pass for one whose clones compile.
+        var synchronized = result.Results
+            .Single(r => r.Generator.GetGeneratorType() == typeof(SyncMethodSourceGenerator))
+            .GeneratedSources
+            .Count(s => s.HintName is not ($"{SyncMethodSourceGenerator.CreateSyncVersionAttribute}.g.cs" or $"{SyncMethodSourceGenerator.SkipSyncVersionAttribute}.g.cs"));
+
+        if (identity.GeneratedSources.Length < synchronized)
+        {
+            throw new InvalidOperationException(
+                $"Identity clone produced {identity.GeneratedSources.Length} files for {synchronized} synchronized methods");
+        }
+
+        var errors = identity.Diagnostics
+            .Concat(compilation.AddSyntaxTrees(result.GeneratedTrees).GetDiagnostics())
+            .Where(d => d.Severity == DiagnosticSeverity.Error)
+            .ToArray();
+
+        if (errors.Length > 0)
+        {
+            throw new InvalidOperationException(
+                "Identity clone does not compile:\n" + string.Join("\n", errors.Select(d => d.ToString())));
+        }
+    }
+
 #if NET8_0_OR_GREATER
     [GeneratedRegex("\r\n")]
     private static partial Regex NewLineRegex();