C# 14: User defined compound assignment operators.

[michaelnebel]

In this PR we introduce support for user-defined compound assignment operators.

class A {
    // Compound operator declaration.
    public void operator +=(A x) {
       ...
    }
}

class B {
    public void M(A a1, A a2) {
        a1 += a2; // Compound operator invocation.
    }
}

Compound assignment operator calls are not implicitly assignments, but instead the left hand side of the compound assignment is updated. That is, user-defined compound operator calls behave more like instance method calls than static operator calls.
Had we instead declared

class A {
    public static A operator +(A x1, A x2) {
        ...
    }
}

then a1 += a2 is also valid, but instead it means a1 = a1 + a2 (where the right hand side is a call to the static operator).

As a consequence, we don't want to expand a1 += a2 into a1 = a1 + a2 in case of user-defined compound operator invocations. Instead we want to extract the syntax as is and consider these type of invocations more like method calls in the QL library code.

Notes on the implementation

• Even though it is claimed that there is some support for dynamic operator calls, I could not produce a running example (and thus haven’t introduced dispatch logic for this case).
• There will be a separate PR for the increment/decrement operator.

[michaelnebel]

DCA looks good

• Performance doesn't appear to be affected.
• Fewer extraction errors.

[Copilot]

Pull request overview

This PR adds C# 14 support for user-defined compound assignment operators (for example a += b where operator += is declared) by extracting them as operator invocations rather than desugaring them into a = a + b, and updates QL dispatch/dataflow plus tests accordingly.

Changes:

• Add new extractor handling for compound assignment expressions to emit direct operator-call extraction for user-defined (instance) compound assignment operators.
• Extend the C# QL library to represent and dispatch CompoundAssignmentOperatorCall distinctly from ordinary OperatorCall.
• Update/extend AST and dataflow library tests and expected outputs for compound assignment operator declarations and invocations (including extension-based operators).

Reviewed changes

Copilot reviewed 19 out of 20 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
csharp/ql/test/library-tests/operators/operators.cs	Adds user-defined compound assignment operator declarations and invocations for AST coverage.
csharp/ql/test/library-tests/operators/PrintAst.expected	Updates AST expectations to include compound assignment operators/calls.
csharp/ql/test/library-tests/operators/Operators1.expected	Updates operator location expectations after AST shifts.
csharp/ql/test/library-tests/operators/Operators2.expected	Updates operator location expectations after AST shifts.
csharp/ql/test/library-tests/operators/Operators3.expected	Updates conversion-operator location expectations after AST shifts.
csharp/ql/test/library-tests/operators/Operators4.expected	Updates conversion-operator location expectations after AST shifts.
csharp/ql/test/library-tests/dataflow/operators/Operator.cs	Adds a dataflow test for user-defined compound assignment operator calls.
csharp/ql/test/library-tests/dataflow/operators/operatorFlow.expected	Updates expected dataflow graph to reflect compound assignment operator modeling.
csharp/ql/test/library-tests/dataflow/extensions/extensions.cs	Adds an extension-based compound assignment operator and a flow test covering it.
csharp/ql/test/library-tests/dataflow/extensions/ExtensionFlow.expected	Updates expected dataflow output for the new extension compound assignment test.
csharp/ql/lib/semmle/code/csharp/exprs/Call.qll	Introduces CompoundAssignmentOperatorCall in the QL expression hierarchy.
csharp/ql/lib/semmle/code/csharp/dispatch/Dispatch.qll	Adjusts dispatch to treat compound assignment operator calls as dispatchable calls with a qualifier.
csharp/ql/lib/semmle/code/csharp/Callable.qll	Adds QL operator classes for compound assignment operator declarations.
csharp/ql/lib/change-notes/2026-03-06-compound-assignment-operations.md	Adds changelog entry for the new C# 14 support.
csharp/extractor/Semmle.Extraction.CSharp/Entities/Expressions/UserCompoundAssignmentInvocation.cs	New extractor expression node for user-defined compound assignment operator invocations.
csharp/extractor/Semmle.Extraction.CSharp/Entities/Expressions/Invocation.cs	Refactors call target resolution to use a shared helper.
csharp/extractor/Semmle.Extraction.CSharp/Entities/Expressions/Factory.cs	Routes compound assignment syntax kinds through new compound-assignment extraction logic.
csharp/extractor/Semmle.Extraction.CSharp/Entities/Expressions/CompoundAssignment.cs	New helper to decide between desugaring vs direct operator-call extraction for compound assignments.
csharp/extractor/Semmle.Extraction.CSharp/CodeAnalysisExtensions/SymbolExtensions.cs	Adds ExpressionNodeInfo.GetTargetSymbol helper for consistent target resolution.
csharp/extractor/Semmle.Extraction.CSharp.Util/SymbolExtensions.cs	Extends operator-symbol parsing to recognize checked and assignment operator method names.

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[Copilot]

Minor wording/grammar: “or a (CompoundAssignmentOperator).” reads oddly and is missing the noun. Consider changing to “or a compound assignment operator (CompoundAssignmentOperator).”

Suggested change

	* a (`CompoundAssignmentOperator`).
	* a compound assignment operator (`CompoundAssignmentOperator`).

[Copilot]

The example in this doc comment appears to be invalid C#: the operator is declared to return void, but the sample then uses return this += other;. Consider adjusting the example to either not return the compound assignment, or to show a (valid) return pattern that matches the operator’s return type.

[Copilot]

Grammar nit: “The following operations doesn't do anything.” → “The following operations don't do anything.”

Suggested change

	// The following operations doesn't do anything.
	// The following operations don't do anything.