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Improve compiler error recovery for null coalesce assignment operator #80907

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Improve compiler error recovery for null coalesce assignment operator #80907

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21 changes: 18 additions & 3 deletions src/Compilers/CSharp/Portable/Binder/Binder_Operators.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -5631,17 +5631,32 @@ private BoundExpression BindNullCoalescingAssignmentOperator(AssignmentExpressio
ReportSuppressionIfNeeded(leftOperand, diagnostics);
BoundExpression rightOperand = BindValue(node.Right, diagnostics, BindValueKind.RValue);

// If either operand is bad, bail out preventing more cascading errors
if (leftOperand.HasAnyErrors || rightOperand.HasAnyErrors)
TypeSymbol leftType = leftOperand.Type;

var rightOperandTargetType = leftType switch
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var rightOperandTargetType = leftType switch

We probably don't want to do this calculation for success scenarios.

{
{ IsReferenceType: true } or { IsValueType: false, TypeKind: TypeKind.TypeParameter } => leftType,
{ IsValueType: true } when leftType.IsNullableType() => leftType.GetNullableUnderlyingType(),
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@AlekseyTs AlekseyTs Oct 27, 2025 •
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leftType.GetNullableUnderlyingType()

Perhaps this would make sense when we are dealing with target-typed new, but I am not sure about other forms of the right operand.

_ => null
};

// If left operand is bad or right operator is bad and we cannot determine its type, take the default error recovery path
if (leftOperand.HasAnyErrors || (rightOperandTargetType is null && rightOperand.HasAnyErrors))
{
leftOperand = BindToTypeForErrorRecovery(leftOperand);
rightOperand = BindToTypeForErrorRecovery(rightOperand);
return new BoundNullCoalescingAssignmentOperator(node, leftOperand, rightOperand, CreateErrorType(), hasErrors: true);
}
// If right operand is bad, but we know its type, make sure conversion is in place for better error recovery
else if (rightOperand.HasAnyErrors)
{
leftOperand = BindToTypeForErrorRecovery(leftOperand);
var conversion = GenerateConversionForAssignment(rightOperandTargetType, rightOperand, diagnostics, ConversionForAssignmentFlags.CompoundAssignment);
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diagnostics

Let's suppress errors from this recovery.

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GenerateConversionForAssignment

Perhaps this would make sense when we are dealing with target-typed new, but I am not sure about other forms of the right operand.

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GenerateConversionForAssignment

We probably can pass the type to BindToTypeForErrorRecovery to get the same effect.

return new BoundNullCoalescingAssignmentOperator(node, leftOperand, conversion, rightOperandTargetType, hasErrors: true);
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rightOperandTargetType

Why is this the right value to use?

}

// Given a ??= b, the type of a is A, the type of B is b, and if A is a nullable value type, the underlying
// non-nullable value type of A is A0.
TypeSymbol leftType = leftOperand.Type;
Debug.Assert((object)leftType != null);

// If A is a non-nullable value type, a compile-time error occurs
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