Fix nullability assertion failure with extension Add methods in collection expressions

[Copilot]

The nullability walker fails with an assertion error when analyzing collection expressions that use extension Add methods:

#nullable enable
object? oNull = null;
MyCollection c = [oNull, oNotNull];

static class E {
    public static void Add(this MyCollection c, object o) { }
}

Error: System.InvalidOperationException: Did not find Microsoft.CodeAnalysis.CSharp.BoundLocal 'oNull' in the map.

Root Cause

For extension methods, NullableWalker only visits Arguments[1] (the element argument), but never visits Arguments[0] (the receiver). When DebugVerifier walks all arguments, it asserts on the untracked receiver.

Changes

• NullableWalker.cs: Mark receiver argument with unknown nullability for extension Add methods in collection expressions
• ExtensionTests.cs: Add regression test

Relates to #68786

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Assertion in nullability analysis of collection expression with extension Add method</issue_title>
<issue_description>The following code results in an assertion when calling VerifyEmitDiagnostics.
This should have been covered by PR #70192
Relates to nullability work for collection expressions: #68786

#nullable enable
using System.Collections;
using System.Collections.Generic;

object? oNull = null;
object oNotNull = new object();
MyCollection c = [oNull, oNotNull];

static class E
{
    public static void Add(this MyCollection c, object o) { }
}

public class MyCollection : IEnumerable<object>
{
    IEnumerator<object> IEnumerable<object>.GetEnumerator() => throw null!;
    IEnumerator IEnumerable.GetEnumerator() => throw null!;
}

  Message: 
System.InvalidOperationException : Did not find Microsoft.CodeAnalysis.CSharp.BoundLocal `oNull` in the map.

  Stack Trace: 
ThrowingTraceListener.Fail(String message, String detailMessage) line 26
TraceInternal.Fail(String message, String detailMessage)
Debug.Fail(String message, String detailMessage)
RoslynDebug.Assert(Boolean condition, AssertInterpolatedStringHandler& message) line 30
DebugVerifier.VerifyExpression(BoundExpression expression, Boolean overrideSkippedExpression) line 75
DebugVerifier.VisitExpressionOrPatternWithoutStackGuard(BoundNode node) line 84
BoundTreeVisitor.VisitExpressionOrPatternWithStackGuard(Int32& recursionDepth, BoundNode node) line 213
DebugVerifier.Visit(BoundNode node) line 100
BoundTreeWalker.VisitList[T](ImmutableArray`1 list) line 23
BoundTreeWalker.VisitCollectionElementInitializer(BoundCollectionElementInitializer node) line 10579
BoundCollectionElementInitializer.Accept(BoundTreeVisitor visitor) line 6893
BoundTreeVisitor.Visit(BoundNode node) line 151
DebugVerifier.VisitExpressionOrPatternWithoutStackGuard(BoundNode node) line 86
BoundTreeVisitor.VisitExpressionOrPatternWithStackGuard(Int32& recursionDepth, BoundNode node) line 213
DebugVerifier.Visit(BoundNode node) line 100
DebugVerifier.VisitCollectionExpression(BoundCollectionExpression node) line 146
BoundCollectionExpression.Accept(BoundTreeVisitor visitor) line 6461
BoundTreeVisitor.Visit(BoundNode node) line 151
DebugVerifier.VisitExpressionOrPatternWithoutStackGuard(BoundNode node) line 86
BoundTreeVisitor.VisitExpressionOrPatternWithStackGuard(Int32& recursionDepth, BoundNode node) line 213
DebugVerifier.Visit(BoundNode node) line 100
BoundTreeWalker.VisitConversion(BoundConversion node) line 10076
DebugVerifier.VisitConversion(BoundConversion node) line 408
BoundConversion.Accept(BoundTreeVisitor visitor) line 2978
BoundTreeVisitor.Visit(BoundNode node) line 151
DebugVerifier.VisitExpressionOrPatternWithoutStackGuard(BoundNode node) line 86
BoundTreeVisitor.VisitExpressionOrPatternWithStackGuard(Int32& recursionDepth, BoundNode node) line 213
DebugVerifier.Visit(BoundNode node) line 100
BoundTreeWalker.VisitLocalDeclaration(BoundLocalDeclaration node) line 10127
BoundLocalDeclaration.Accept(BoundTreeVisitor visitor) line 3419
BoundTreeVisitor.Visit(BoundNode node) line 151
DebugVerifier.Visit(BoundNode node) line 102
BoundTreeWalker.VisitList[T](ImmutableArray`1 list) line 23
BoundTreeWalker.VisitBlock(BoundBlock node) line 10111
BoundBlock.Accept(BoundTreeVisitor visitor) line 3323
BoundTreeVisitor.Visit(BoundNode node) line 151
DebugVerifier.Visit(BoundNode node) line 102
BoundTreeWalker.VisitNonConstructorMethodBody(BoundNonConstructorMethodBody node) line 10808
BoundNonConstructorMethodBody.Accept(BoundTreeVisitor visitor) line 8749
BoundTreeVisitor.Visit(BoundNode node) line 151
DebugVerifier.Visit(BoundNode node) line 102
DebugVerifier.Verify(ImmutableDictionary`2 analyzedNullabilityMap, SnapshotManager snapshotManagerOpt, BoundNode node) line 40
NullableWalker.AnalyzeWithSemanticInfo(CSharpCompilation compilation, Symbol symbol, BoundNode node, Binder binder, VariableState initialState, DiagnosticBag diagnostics, Boolean createSnapshots, Boolean requiresAnalysis) line 1634
NullableWalker.AnalyzeAndRewrite(CSharpCompilation compilation, Symbol symbol, BoundNode node, Binder binder, VariableState initialState, DiagnosticBag diagnostics, Boolean createSnapshots, SnapshotManager& snapshotManager, ImmutableDictionary`2& remappedSymbols) line 1586
MethodCompiler.BindMethodBody(MethodSymbol method, TypeCompilationState compilationState, BindingDiagnosticBag diagnostics, Boolean includeInitializersInBody, BoundNode initializersBody, Boolean reportNullableDiagnostics, ImportChain& importChain, Boolean& originalBodyNested, Boolean& prependedDefaultValueTypeConstructorInitializer, InitialState& forSemanticModel) line 1910
MethodCompiler.CompileMethod(MethodSymbol methodSymbol, Int32 methodOrdinal, ProcessedFieldInitializers& processedInitializers, SynthesizedSubmissionFields previous...

</details>

- Fixes dotnet/roslyn#78452

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