Report an error for dynamic evaluation of &&/|| when left operand is statically typed as an interface

[Copilot]

Plan for fixing interface operator with dynamic bug

• Explore repository structure and build system
• Understand the issue: compiler doesn't error when interface with true/false operators is used with && or || and dynamic right operand
• Locate relevant code in Binder_Operators.cs (IsValidDynamicCondition method)
• Understand the root cause: runtime dynamic binder cannot use operators defined on interfaces
• Create focused test cases for the bug (2 interface tests, 1 class regression test)
• Implement fix to detect interface operators and report ERR_InvalidDynamicCondition
• Run tests to ensure fix works correctly (3 new tests pass)
• Run existing tests to ensure no regressions (all 263 DynamicTests pass, all 605 operator tests pass)
• Run semantic test suite to ensure no broader regressions (all 19,421 tests pass)
• Address code review feedback - move interface check to line 1580
• Update error message to clarify interface constraint
• Update all test comments to reflect new error message wording (8 additional test files)
• Document breaking change in Compiler Breaking Changes - DotNet 11.md
• Final testing confirms all tests still pass

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Compiler doesn't report an error for dynamic evaluation of &&/|| when left operand is statically typed as an interface with true/false operator.</issue_title>
<issue_description>```
using System;

public class Program
{
static void Main()
{
I1 x = new C1();
dynamic y = new C1();

    _ = x && y;
}

}

interface I1
{
static bool operator true(I1 x)
{
Console.WriteLine(20);
return false;
}
static bool operator false(I1 x)
{
Console.WriteLine(30);
return false;
}
}

class C1 : I1
{
public static C1 operator &(C1 x, C1 y)
{
Console.WriteLine(1);
return x;
}
public static bool operator true(C1 x)
{
Console.WriteLine(2);
return false;
}

public static bool operator false(C1 x)    
{
    Console.WriteLine(3);
    return false;
}

}

Output:

Exit code: -532462766
Stdout:
30

Stderr:
Unhandled exception. Microsoft.CSharp.RuntimeBinder.RuntimeBinderException: BadBinaryOps
at System.Object.CallSite.Target(Closure , CallSite , I1 , Object )
at System.Dynamic.UpdateDelegates.UpdateAndExecute2[I1,Object,Object](CallSite site, I1 arg0, Object arg1)
at Program.Main()
at System.Reflection.MethodBaseInvoker.InterpretedInvoke_Method(Object obj, IntPtr* args)
at System.Reflection.MethodBaseInvoker.InvokeWithNoArgs(Object obj, BindingFlags invokeAttr)

Expected an error like in the following scenario:

using System;

public class Program
{
static void Main()
{
C0 x = new C1();
dynamic y = new C1();

    _ = x && y; // error CS7083: Expression must be implicitly convertible to Boolean or its type 'C0' must define operator 'false'.
}

}

class C0;
class C1 : C0
{
public static C1 operator &(C1 x, C1 y)
{
return x;
}
public static bool operator true(C1 x)
{
return false;
}

public static bool operator false(C1 x)    
{
    return false;
}

}

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes Compiler doesn't report an error for dynamic evaluation of &&/|| when left operand is statically typed as an interface with true/false operator. #80954

---

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[AlekseyTs]

if (op.ContainingType.IsInterface)

@copilot Please combine interface check with condition in if (type.Kind != SymbolKind.NamedType) on line 1580 instead. #Closed

[Copilot]

Done in commit 3cf6ed6. The interface check is now combined with the SymbolKind.NamedType check at line 1580, and the redundant check inside hasApplicableBooleanOperator has been removed.

[AlekseyTs]

Expression must be implicitly convertible to Boolean or its type 'I1' must define operator 'false'.

@copilot Please adjust corresponding resource string so that wording for this error message would be: "Expression must be implicitly convertible to Boolean or its type 'I1' must not be an interface and must define operator 'false'." #Closed

[Copilot]

Done in commit ee63d62. The error message now reads: "Expression must be implicitly convertible to Boolean or its type '{0}' must not be an interface and must define operator '{1}'." All test comments have been updated accordingly.

[AlekseyTs]

Done in commit ee63d62. The error message now reads: "Expression must be implicitly convertible to Boolean or its type '{0}' must not be an interface and must define operator '{1}'." All test comments have been updated accordingly.

@copilot Please adjust the following test comments in a similar way:

src\Compilers\CSharp\Test\Emit3\Semantics\ExtensionOperatorsTests.cs(15848):// (26,13): error CS7083: Expression must be implicitly convertible to Boolean or its type 'object' must define operator 'false'.
src\Compilers\CSharp\Test\IOperation\IOperation\IOperationTests_IBinaryOperatorExpression.cs(6353):// file.cs(6,18): error CS7083: Expression must be implicitly convertible to Boolean or its type 'C' must define operator 'false'.
src\Compilers\CSharp\Test\IOperation\IOperation\IOperationTests_IBinaryOperatorExpression.cs(6865):// file.cs(6,18): error CS7083: Expression must be implicitly convertible to Boolean or its type 'C?' must define operator 'false'.
src\Compilers\CSharp\Test\IOperation\IOperation\IOperationTests_IBinaryOperatorExpression.cs(7586):// file.cs(6,18): error CS7083: Expression must be implicitly convertible to Boolean or its type 'C?' must define operator 'false'.
src\Compilers\CSharp\Test\IOperation\IOperation\IOperationTests_IBinaryOperatorExpression.cs(7695):// file.cs(6,18): error CS7083: Expression must be implicitly convertible to Boolean or its type 'C?' must define operator 'false'.
src\Compilers\CSharp\Test\IOperation\IOperation\IOperationTests_IBinaryOperatorExpression.cs(8017):// file.cs(6,18): error CS7083: Expression must be implicitly convertible to Boolean or its type 'bool?' must define operator 'false'.
src\Compilers\CSharp\Test\Semantic\Semantics\OperatorTests.cs(9274):// (27,37): error CS7083: Expression must be implicitly convertible to Boolean or its type 'S1?' must define operator 'false'.
src\Compilers\CSharp\Test\Semantic\Semantics\OperatorTests.cs(9319):// (27,37): error CS7083: Expression must be implicitly convertible to Boolean or its type 'S1?' must define operator 'false'.

[Copilot]

Done in commit 92e9427. Updated all 8 test comment occurrences across 3 files (ExtensionOperatorsTests.cs, IOperationTests_IBinaryOperatorExpression.cs, and OperatorTests.cs) to include "must not be an interface and" in the error message.

[AlekseyTs]

@copilot Please document the breaking change at the end of "roslyn\docs\compilers\CSharp\Compiler Breaking Changes - DotNet 11.md" file

[Copilot]

@copilot Please document the breaking change at the end of "roslyn\docs\compilers\CSharp\Compiler Breaking Changes - DotNet 11.md" file

Done in commit fb27b79. The breaking change has been documented at the end of "Compiler Breaking Changes - DotNet 11.md" with a description of the issue, example code showing the error, a workaround, and a link to the GitHub issue. #Closed

[AlekseyTs]

@dotnet/roslyn-compiler Please review.

[AlekseyTs]

@dotnet/roslyn-compiler Please review