An assert is firing for scenario in BadIteratorReturnInRefReturningMethod unit-test.

[AlekseyTs]

Add .GetEmitDiagnostics(); at the end of the Microsoft.CodeAnalysis.CSharp.UnitTests.Semantics.RefLocalsAndReturnsTests.BadIteratorReturnInRefReturningMethod unit-test and run it.

Observed:

 	Roslyn.Test.Utilities.dll!Microsoft.CodeAnalysis.ThrowingTraceListener.Fail(string message, string detailMessage) Line 24	C#
 	System.dll!System.Diagnostics.TraceListener.Fail(string message) Line 181	C#
 	System.dll!System.Diagnostics.TraceInternal.Fail(string message) Line 225	C#
 	System.dll!System.Diagnostics.Debug.Assert(bool condition) Line 100	C#
 	Microsoft.CodeAnalysis.CSharp.dll!Microsoft.CodeAnalysis.CSharp.SyntheticBoundNodeFactory.Return(Microsoft.CodeAnalysis.CSharp.BoundExpression expression) Line 468	C#
 	Microsoft.CodeAnalysis.CSharp.dll!Microsoft.CodeAnalysis.CSharp.IteratorRewriter.GenerateEnumeratorImplementation() Line 231	C#
 	Microsoft.CodeAnalysis.CSharp.dll!Microsoft.CodeAnalysis.CSharp.IteratorRewriter.GenerateMethodImplementations() Line 179	C#
 	Microsoft.CodeAnalysis.CSharp.dll!Microsoft.CodeAnalysis.CSharp.StateMachineRewriter.Rewrite() Line 120	C#
 	Microsoft.CodeAnalysis.CSharp.dll!Microsoft.CodeAnalysis.CSharp.IteratorRewriter.Rewrite(Microsoft.CodeAnalysis.CSharp.BoundStatement body, Microsoft.CodeAnalysis.CSharp.Symbols.MethodSymbol method, int methodOrdinal, Microsoft.CodeAnalysis.CodeGen.VariableSlotAllocator slotAllocatorOpt, Microsoft.CodeAnalysis.CSharp.TypeCompilationState compilationState, Microsoft.CodeAnalysis.DiagnosticBag diagnostics, out Microsoft.CodeAnalysis.CSharp.IteratorStateMachine stateMachineType) Line 84	C#
 	Microsoft.CodeAnalysis.CSharp.dll!Microsoft.CodeAnalysis.CSharp.MethodCompiler.LowerBodyOrInitializer(Microsoft.CodeAnalysis.CSharp.Symbols.MethodSymbol method, int methodOrdinal, Microsoft.CodeAnalysis.CSharp.BoundStatement body, Microsoft.CodeAnalysis.CSharp.SynthesizedSubmissionFields previousSubmissionFields, Microsoft.CodeAnalysis.CSharp.TypeCompilationState compilationState, bool instrumentForDynamicAnalysis, Microsoft.CodeAnalysis.CodeGen.DebugDocumentProvider debugDocumentProvider, ref System.Collections.Immutable.ImmutableArray<Microsoft.CodeAnalysis.CodeGen.SourceSpan> dynamicAnalysisSpans, Microsoft.CodeAnalysis.DiagnosticBag diagnostics, ref Microsoft.CodeAnalysis.CodeGen.VariableSlotAllocator lazyVariableSlotAllocator, Microsoft.CodeAnalysis.PooledObjects.ArrayBuilder<Microsoft.CodeAnalysis.CodeGen.LambdaDebugInfo> lambdaDebugInfoBuilder, Microsoft.CodeAnalysis.PooledObjects.ArrayBuilder<Microsoft.CodeAnalysis.CodeGen.ClosureDebugInfo> closureDebugInfoBuilder, out Microsoft.CodeAnalysis.CSharp.StateMachineTypeSymbol stateMachineTypeOpt) Line 1380	C#
 	Microsoft.CodeAnalysis.CSharp.dll!Microsoft.CodeAnalysis.CSharp.MethodCompiler.CompileMethod(Microsoft.CodeAnalysis.CSharp.Symbols.MethodSymbol methodSymbol, int methodOrdinal, ref Microsoft.CodeAnalysis.CSharp.Binder.ProcessedFieldInitializers processedInitializers, Microsoft.CodeAnalysis.CSharp.SynthesizedSubmissionFields previousSubmissionFields, Microsoft.CodeAnalysis.CSharp.TypeCompilationState compilationState) Line 1138	C#
 	Microsoft.CodeAnalysis.CSharp.dll!Microsoft.CodeAnalysis.CSharp.MethodCompiler.CompileNamedType(Microsoft.CodeAnalysis.CSharp.Symbols.NamedTypeSymbol containingType) Line 518	C#
 	Microsoft.CodeAnalysis.CSharp.dll!Microsoft.CodeAnalysis.CSharp.MethodCompiler.VisitNamedType(Microsoft.CodeAnalysis.CSharp.Symbols.NamedTypeSymbol symbol, Microsoft.CodeAnalysis.CSharp.TypeCompilationState arg) Line 396	C#
 	Microsoft.CodeAnalysis.CSharp.dll!Microsoft.CodeAnalysis.CSharp.Symbols.NamedTypeSymbol.Accept<Microsoft.CodeAnalysis.CSharp.TypeCompilationState, object>(Microsoft.CodeAnalysis.CSharp.CSharpSymbolVisitor<Microsoft.CodeAnalysis.CSharp.TypeCompilationState, object> visitor, Microsoft.CodeAnalysis.CSharp.TypeCompilationState argument) Line 572	C#
 	Microsoft.CodeAnalysis.CSharp.dll!Microsoft.CodeAnalysis.CSharp.MethodCompiler.CompileNamespace(Microsoft.CodeAnalysis.CSharp.Symbols.NamespaceSymbol symbol) Line 375	C#
 	Microsoft.CodeAnalysis.CSharp.dll!Microsoft.CodeAnalysis.CSharp.MethodCompiler.CompileMethodBodies(Microsoft.CodeAnalysis.CSharp.CSharpCompilation compilation, Microsoft.CodeAnalysis.CSharp.Emit.PEModuleBuilder moduleBeingBuiltOpt, bool emittingPdb, bool emitTestCoverageData, bool hasDeclarationErrors, bool emitMethodBodies, Microsoft.CodeAnalysis.DiagnosticBag diagnostics, System.Predicate<Microsoft.CodeAnalysis.CSharp.Symbol> filterOpt, System.Threading.CancellationToken cancellationToken) Line 156	C#
>	Microsoft.CodeAnalysis.CSharp.dll!Microsoft.CodeAnalysis.CSharp.CSharpCompilation.CompileMethods(Microsoft.CodeAnalysis.Emit.CommonPEModuleBuilder moduleBuilder, bool emittingPdb, bool emitMetadataOnly, bool emitTestCoverageData, Microsoft.CodeAnalysis.DiagnosticBag diagnostics, System.Predicate<Microsoft.CodeAnalysis.Symbols.ISymbolInternal> filterOpt, System.Threading.CancellationToken cancellationToken) Line 2771	C#
 	Microsoft.CodeAnalysis.dll!Microsoft.CodeAnalysis.Compilation.Emit(System.IO.Stream peStream, System.IO.Stream metadataPEStream, System.IO.Stream pdbStream, System.IO.Stream xmlDocumentationStream, System.IO.Stream win32Resources, System.Collections.Generic.IEnumerable<Microsoft.CodeAnalysis.ResourceDescription> manifestResources, Microsoft.CodeAnalysis.Emit.EmitOptions options, Microsoft.CodeAnalysis.IMethodSymbol debugEntryPoint, System.IO.Stream sourceLinkStream, System.Collections.Generic.IEnumerable<Microsoft.CodeAnalysis.EmbeddedText> embeddedTexts, Microsoft.CodeAnalysis.CodeGen.CompilationTestData testData, System.Threading.CancellationToken cancellationToken) Line 2497	C#
 	Microsoft.CodeAnalysis.dll!Microsoft.CodeAnalysis.Compilation.Emit(System.IO.Stream peStream, System.IO.Stream pdbStream, System.IO.Stream xmlDocumentationStream, System.IO.Stream win32Resources, System.Collections.Generic.IEnumerable<Microsoft.CodeAnalysis.ResourceDescription> manifestResources, Microsoft.CodeAnalysis.Emit.EmitOptions options, Microsoft.CodeAnalysis.IMethodSymbol debugEntryPoint, System.IO.Stream sourceLinkStream, System.Collections.Generic.IEnumerable<Microsoft.CodeAnalysis.EmbeddedText> embeddedTexts, System.IO.Stream metadataPEStream, System.Threading.CancellationToken cancellationToken) Line 2441	C#
 	Roslyn.Test.Utilities.dll!Microsoft.CodeAnalysis.DiagnosticExtensions.GetEmitDiagnostics<Microsoft.CodeAnalysis.CSharp.CSharpCompilation>(Microsoft.CodeAnalysis.CSharp.CSharpCompilation c, Microsoft.CodeAnalysis.Emit.EmitOptions options, System.Collections.Generic.IEnumerable<Microsoft.CodeAnalysis.ResourceDescription> manifestResources) Line 376	C#
 	Roslyn.Test.Utilities.dll!Microsoft.CodeAnalysis.DiagnosticExtensions.GetEmitDiagnostics<Microsoft.CodeAnalysis.CSharp.CSharpCompilation>(Microsoft.CodeAnalysis.CSharp.CSharpCompilation c) Line 388	C#
 	Microsoft.CodeAnalysis.CSharp.Test.Utilities.dll!Microsoft.CodeAnalysis.CSharp.Test.Utilities.CSharpTestBase.VerifyUsedAssemblyReferences(System.Func<Microsoft.CodeAnalysis.CSharp.CSharpCompilation> createCompilationLambda) Line 1061	C#
 	Microsoft.CodeAnalysis.CSharp.Test.Utilities.dll!Microsoft.CodeAnalysis.CSharp.Test.Utilities.CSharpTestBase.CreateCompilationCore(Microsoft.CodeAnalysis.CSharp.Test.Utilities.CSharpTestSource source, System.Collections.Generic.IEnumerable<Microsoft.CodeAnalysis.MetadataReference> references, Microsoft.CodeAnalysis.CSharp.CSharpCompilationOptions options, Microsoft.CodeAnalysis.CSharp.CSharpParseOptions parseOptions, string assemblyName, string sourceFileName, bool skipUsesIsNullable, Microsoft.CodeAnalysis.CSharp.MessageID? experimentalFeature) Line 1050	C#
 	Microsoft.CodeAnalysis.CSharp.Test.Utilities.dll!Microsoft.CodeAnalysis.CSharp.Test.Utilities.CSharpTestBase.CreateEmptyCompilation(Microsoft.CodeAnalysis.CSharp.Test.Utilities.CSharpTestSource source, System.Collections.Generic.IEnumerable<Microsoft.CodeAnalysis.MetadataReference> references, Microsoft.CodeAnalysis.CSharp.CSharpCompilationOptions options, Microsoft.CodeAnalysis.CSharp.CSharpParseOptions parseOptions, string assemblyName, string sourceFileName, bool skipUsesIsNullable) Line 1008	C#
 	Microsoft.CodeAnalysis.CSharp.Test.Utilities.dll!Microsoft.CodeAnalysis.CSharp.Test.Utilities.CSharpTestBase.CreateCompilation(Microsoft.CodeAnalysis.CSharp.Test.Utilities.CSharpTestSource source, System.Collections.Generic.IEnumerable<Microsoft.CodeAnalysis.MetadataReference> references, Microsoft.CodeAnalysis.CSharp.CSharpCompilationOptions options, Microsoft.CodeAnalysis.CSharp.CSharpParseOptions parseOptions, Roslyn.Test.Utilities.TargetFramework targetFramework, string assemblyName, string sourceFileName, bool skipUsesIsNullable) Line 999	C#
 	Microsoft.CodeAnalysis.CSharp.Test.Utilities.dll!Microsoft.CodeAnalysis.CSharp.Test.Utilities.CSharpTestBase.CreateCompilationWithMscorlib46(Microsoft.CodeAnalysis.CSharp.Test.Utilities.CSharpTestSource source, System.Collections.Generic.IEnumerable<Microsoft.CodeAnalysis.MetadataReference> references, Microsoft.CodeAnalysis.CSharp.CSharpCompilationOptions options, Microsoft.CodeAnalysis.CSharp.CSharpParseOptions parseOptions, string assemblyName, string sourceFileName) Line 904	C#
 	Microsoft.CodeAnalysis.CSharp.Semantic.UnitTests.dll!Microsoft.CodeAnalysis.CSharp.UnitTests.Semantics.RefLocalsAndReturnsTests.BadIteratorReturnInRefReturningMethod() Line 3468	C#

We probably shouldn't do lowering in this scenario since we have rather serious binding errors:

// (7,28): error CS8089: The body of 'C.ObjEnumerator()' cannot be an iterator block because 'C.ObjEnumerator()' returns by reference
//     public ref IEnumerator ObjEnumerator()
Diagnostic(ErrorCode.ERR_BadIteratorReturnRef, "ObjEnumerator").WithArguments("C.ObjEnumerator()").WithLocation(7, 28),
// (12,28): error CS8089: The body of 'C.ObjEnumerable()' cannot be an iterator block because 'C.ObjEnumerable()' returns by reference
//     public ref IEnumerable ObjEnumerable()
Diagnostic(ErrorCode.ERR_BadIteratorReturnRef, "ObjEnumerable").WithArguments("C.ObjEnumerable()").WithLocation(12, 28),
// (17,33): error CS8089: The body of 'C.GenEnumerator()' cannot be an iterator block because 'C.GenEnumerator()' returns by reference
//     public ref IEnumerator<int> GenEnumerator()
Diagnostic(ErrorCode.ERR_BadIteratorReturnRef, "GenEnumerator").WithArguments("C.GenEnumerator()").WithLocation(17, 33),
// (22,33): error CS8089: The body of 'C.GenEnumerable()' cannot be an iterator block because 'C.GenEnumerable()' returns by reference
//     public ref IEnumerable<int> GenEnumerable()
Diagnostic(ErrorCode.ERR_BadIteratorReturnRef, "GenEnumerable").WithArguments("C.GenEnumerable()").WithLocation(22, 33));