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Merged
merged 7 commits into from
Aug 8, 2025
Merged

Loosen casting requirements on switch expression conversion #77139

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52e6713
Loosen casting requirements on switch expression conversion
RenderMichael Feb 10, 2025
595ed10
Merge remote-tracking branch 'upstream/main' into switch-expression-cast
CyrusNajmabadi Aug 8, 2025
b896ae0
In progress
CyrusNajmabadi Aug 8, 2025
06488b3
Add cast removal support
CyrusNajmabadi Aug 8, 2025
3081581
Revert
CyrusNajmabadi Aug 8, 2025
af6466d
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CyrusNajmabadi Aug 8, 2025
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CyrusNajmabadi Aug 8, 2025
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77 changes: 77 additions & 0 deletions ...Sharp/Tests/ConvertSwitchStatementToExpression/ConvertSwitchStatementToExpressionTests.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -2251,6 +2251,48 @@ void M(string s)
}
""");

[Fact, WorkItem("https://github.com/dotnet/roslyn/issues/77084")]
public async Task TestRuntimeTypeConversion_Assignment3()
{
await VerifyCS.VerifyCodeFixAsync(
"""
class Program
{
void M(string s)
{
object result;

[|switch|] (s)
{
case "a":
result = 1234;
break;
case "b":
result = 3.14;
break;
default:
result = true;
break;
}
}
}
""",
"""
class Program
{
void M(string s)
{
object result = s switch
{
"a" => 1234,
"b" => 3.14,
_ => true,
};
}
}
""");
}

[Fact, WorkItem("https://github.com/dotnet/roslyn/issues/58636")]
public Task TestRuntimeTypeConversion_Return1()
=> VerifyCS.VerifyCodeFixAsync(
Expand Down Expand Up @@ -2324,6 +2366,41 @@ object M(string s)
}
""");

[Fact, WorkItem("https://github.com/dotnet/roslyn/issues/77084")]
public async Task TestRuntimeTypeConversion_Return3()
{
await VerifyCS.VerifyCodeFixAsync(
"""
class Program
{
object M(string s)
{
[|switch|] (s)
{
case "a":
return true;

default:
return false;
}
}
}
""",
"""
class Program
{
object M(string s)
{
return s switch
{
"a" => true,
_ => false,
};
}
}
""");
}

[Fact, WorkItem("https://github.com/dotnet/roslyn/issues/61278")]
public Task TestLeadingTrivia1()
=> new VerifyCS.Test
Expand Down
109 changes: 91 additions & 18 deletions ...SharedUtilitiesAndExtensions/Compiler/CSharp/Simplification/Simplifiers/CastSimplifier.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -3,6 +3,7 @@
// See the LICENSE file in the project root for more information.

using System;
using System.Collections.Immutable;
using System.Diagnostics;
using System.Diagnostics.CodeAnalysis;
using System.Linq;
Expand Down Expand Up @@ -578,6 +579,16 @@ private static bool IsConversionCastSafeToRemove(
return true;
}

// Similarly, we want to support this for:
//
// int? a = b switch { true => (int?)0, false => 1 }
if (IsSwitchExpressionCaseCastSafeToRemove(
castNode, originalSemanticModel,
rewrittenExpression, rewrittenSemanticModel, cancellationToken))
{
return true;
}

// Widening a value before bitwise negation produces the same value as bitwise negation
// followed by the same widening. For example:
//
Expand Down Expand Up @@ -825,17 +836,75 @@ private static bool IsConditionalCastSafeToRemove(
ExpressionSyntax castNode, SemanticModel originalSemanticModel,
ExpressionSyntax rewrittenExpression, SemanticModel rewrittenSemanticModel, CancellationToken cancellationToken)
{
if (castNode is not CastExpressionSyntax castExpression)
return false;
// Defer to common helper to determine if the cast can be removed. This unified processing of `x ? y : z` and
// `x switch { .. => y, .. => z, .. => w, ... }` expressions.
return IsSwitchOrConditionalCastSafeToRemove(
castNode,
originalSemanticModel,
rewrittenExpression,
rewrittenSemanticModel,
static parentExpression => parentExpression.Parent is ConditionalExpressionSyntax conditionalExpression && conditionalExpression.Condition != parentExpression
? conditionalExpression
: null,
static conditionalExpression => [conditionalExpression.WhenTrue, conditionalExpression.WhenFalse],
static (conditionalExpression, armExpression) =>
{
Contract.ThrowIfFalse(conditionalExpression.WhenTrue == armExpression || conditionalExpression.WhenFalse == armExpression);
return armExpression == conditionalExpression.WhenTrue
? conditionalExpression.WhenFalse
: conditionalExpression.WhenTrue;
},
cancellationToken);
}

private static bool IsSwitchExpressionCaseCastSafeToRemove(
ExpressionSyntax castNode, SemanticModel originalSemanticModel,
ExpressionSyntax rewrittenExpression, SemanticModel rewrittenSemanticModel, CancellationToken cancellationToken)
{
// Defer to common helper to determine if the cast can be removed. This unified processing of `x ? y : z` and
// `x switch { .. => y, .. => z, .. => w, ... }` expressions.
return IsSwitchOrConditionalCastSafeToRemove(
castNode,
originalSemanticModel,
rewrittenExpression,
rewrittenSemanticModel,
static parentExpression => parentExpression.Parent is SwitchExpressionArmSyntax { Parent: SwitchExpressionSyntax switchExpression }
? switchExpression
: null,
static switchExpression => switchExpression.Arms.SelectAsArray(a => a.Expression),
static (switchExpression, armExpression) =>
{
if (switchExpression.Arms.Count < 2)
return null;

var arm = switchExpression.Arms.Single(a => a.Expression == armExpression);
var armIndex = switchExpression.Arms.IndexOf(arm);
return armIndex == 0
? switchExpression.Arms[1].Expression
: switchExpression.Arms[armIndex - 1].Expression;
},
cancellationToken);
}

var parent = castExpression.WalkUpParentheses();
if (parent.Parent is not ConditionalExpressionSyntax originalConditionalExpression)
private static bool IsSwitchOrConditionalCastSafeToRemove<TConditionalOrSwitchExpression>(
ExpressionSyntax castNode,
SemanticModel originalSemanticModel,
ExpressionSyntax rewrittenExpression,
SemanticModel rewrittenSemanticModel,
Func<ExpressionSyntax, TConditionalOrSwitchExpression?> getConditionalOrSwitchExpression,
Func<TConditionalOrSwitchExpression, ImmutableArray<ExpressionSyntax>> getArmExpressions,
Func<TConditionalOrSwitchExpression, ExpressionSyntax, ExpressionSyntax?> getAlternativeArm,
CancellationToken cancellationToken)
where TConditionalOrSwitchExpression : ExpressionSyntax
{
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@CyrusNajmabadi CyrusNajmabadi Aug 8, 2025

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all the logic in here is what we had in IsConditionalCastSafeToRemove, but made generic so it can operate on a a?b:c expr vs a a switch { ... } expr, with helper functions to deal with conditional exprs only having two arms, which switches can have many.

if (castNode is not CastExpressionSyntax castExpression)
return false;

// if we were parented by a conditional before, we must be parented by a conditional afterwards.
var rewrittenConditionalExpression = (ConditionalExpressionSyntax)rewrittenExpression.WalkUpParentheses().GetRequiredParent();
var parentExpression = castExpression.WalkUpParentheses();

if (parent != originalConditionalExpression.WhenFalse && parent != originalConditionalExpression.WhenTrue)
var originalConditionalOrSwitchExpression = getConditionalOrSwitchExpression(parentExpression);
var rewrittenConditionalOrSwitchExpression = getConditionalOrSwitchExpression(rewrittenExpression.WalkUpParentheses());
if (originalConditionalOrSwitchExpression is null || rewrittenConditionalOrSwitchExpression is null)
return false;

if (originalSemanticModel.GetOperation(castExpression, cancellationToken) is not IConversionOperation conversionOperation)
Expand All @@ -856,16 +925,16 @@ bool IsConditionalCastSafeToRemoveDueToConversionOfEntireConditionalExpression()
{
// if we have `a ? (int?)b : default` then we can't remove the nullable cast as it changes the
// meaning of `default`.
if (originalConditionalExpression.WhenTrue.WalkDownParentheses().IsKind(SyntaxKind.DefaultLiteralExpression) ||
originalConditionalExpression.WhenFalse.WalkDownParentheses().IsKind(SyntaxKind.DefaultLiteralExpression))
foreach (var armExpression in getArmExpressions(originalConditionalOrSwitchExpression))
{
return false;
if (armExpression.WalkDownParentheses().IsKind(SyntaxKind.DefaultLiteralExpression))
return false;
}
}

var originalCastExpressionTypeInfo = originalSemanticModel.GetTypeInfo(castExpression, cancellationToken);
var originalConditionalTypeInfo = originalSemanticModel.GetTypeInfo(originalConditionalExpression, cancellationToken);
var rewrittenConditionalTypeInfo = rewrittenSemanticModel.GetTypeInfo(rewrittenConditionalExpression, cancellationToken);
var originalConditionalTypeInfo = originalSemanticModel.GetTypeInfo(originalConditionalOrSwitchExpression, cancellationToken);
var rewrittenConditionalTypeInfo = rewrittenSemanticModel.GetTypeInfo(rewrittenConditionalOrSwitchExpression, cancellationToken);

if (IsNullOrErrorType(originalCastExpressionTypeInfo) ||
IsNullOrErrorType(originalConditionalTypeInfo) ||
Expand All @@ -886,13 +955,14 @@ bool IsConditionalCastSafeToRemoveDueToConversionOfEntireConditionalExpression()
if (IsNullOrErrorType(castType))
return false;

if (rewrittenSemanticModel.GetOperation(rewrittenConditionalExpression, cancellationToken) is not IConditionalOperation rewrittenConditionalOperation)
var rewrittenOperation = rewrittenSemanticModel.GetOperation(rewrittenConditionalOrSwitchExpression, cancellationToken);
if (rewrittenOperation is not IConditionalOperation and not ISwitchExpressionOperation)
return false;

if (castType.Equals(rewrittenConditionalOperation.Type, SymbolEqualityComparer.IncludeNullability))
if (castType.Equals(rewrittenOperation.Type, SymbolEqualityComparer.IncludeNullability))
return true;

if (rewrittenConditionalOperation.Parent is IConversionOperation conditionalParentConversion &&
if (rewrittenOperation.Parent is IConversionOperation conditionalParentConversion &&
conditionalParentConversion.GetConversion().IsImplicit &&
castType.Equals(conditionalParentConversion.Type, SymbolEqualityComparer.IncludeNullability))
{
Expand All @@ -911,7 +981,10 @@ bool IsConditionalCastSafeToRemoveDueToConversionToOtherBranch()
if (castExpression.Expression.WalkDownParentheses().IsKind(SyntaxKind.DefaultLiteralExpression))
return false;

var otherSide = parent == originalConditionalExpression.WhenFalse ? originalConditionalExpression.WhenTrue : originalConditionalExpression.WhenFalse;
var otherSide = getAlternativeArm(originalConditionalOrSwitchExpression, parentExpression);
if (otherSide is null)
return false;

var otherSideType = originalSemanticModel.GetTypeInfo(otherSide, cancellationToken).Type;
var thisSideRewrittenType = rewrittenSemanticModel.GetTypeInfo(rewrittenExpression, cancellationToken).Type;

Expand All @@ -925,11 +998,11 @@ bool IsConditionalCastSafeToRemoveDueToConversionToOtherBranch()
// Now check that with the (T) cast removed, that the outer `x ? y : z` is still
// immediately implicitly converted to a 'T'. If so, we can remove this inner (T) cast.

var rewrittenConditionalConvertedType = rewrittenSemanticModel.GetTypeInfo(rewrittenConditionalExpression, cancellationToken).ConvertedType;
var rewrittenConditionalConvertedType = rewrittenSemanticModel.GetTypeInfo(rewrittenConditionalOrSwitchExpression, cancellationToken).ConvertedType;
if (rewrittenConditionalConvertedType is null)
return false;

var outerConversion = rewrittenSemanticModel.GetConversion(rewrittenConditionalExpression, cancellationToken);
var outerConversion = rewrittenSemanticModel.GetConversion(rewrittenConditionalOrSwitchExpression, cancellationToken);
if (!outerConversion.IsImplicit)
return false;

Expand Down
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