fix: Emit string.Concat instead of Expression.Add for string + operator

[koenbeuk]

Summary

• String concatenation via the + operator was incorrectly emitting Expression.MakeBinary(ExpressionType.Add, ...) which is only valid for numeric types — at runtime this throws InvalidOperationException
• Now emits Expression.Call(string.Concat, ...), matching the existing interpolation codepath
• Picks Concat(string, string) when both operands are strings, Concat(object, object) for mixed types (e.g. int + string with boxing)
• Also fixes the same latent bug in EmitCompoundAssignment for string +=
• Safe for overloaded operators: only triggers when OperatorMethod is null (built-in compiler intrinsic)

Test plan

• 3 new snapshot tests (StringPlusString, IntPlusString, SingleStringConcat)
• 14 existing snapshot tests updated (extension member + constructor tests that used string +)
• New integration test (Select_SummaryConcat_ReturnsCorrectValues) verifying runtime correctness
• Full test suite passes (897 tests, 0 failures)

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Fixes expression-tree emission for C# string concatenation so that +/+= producing string are emitted as Expression.Call(string.Concat, ...) rather than Expression.MakeBinary(ExpressionType.Add, ...), preventing runtime InvalidOperationException for string addition scenarios and aligning with the interpolation emission path.

Changes:

• Update generator emission for string + and string += to call string.Concat (string/string vs object/object overload selection).
• Add new generator snapshot tests covering string concatenation variants.
• Add an integration test scenario/property to validate runtime correctness when executing expanded expressions.

Reviewed changes

Copilot reviewed 20 out of 20 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/ExpressiveSharp.Generator/Emitter/ExpressionTreeEmitter.cs	Emits string.Concat for + / += when the result type is string and the operator is intrinsic.
tests/ExpressiveSharp.IntegrationTests/Scenarios/Store/Models/Order.cs	Adds an [Expressive] property using + string concatenation for integration coverage.
tests/ExpressiveSharp.IntegrationTests/Scenarios/Common/Tests/StringOperationTests.cs	Adds integration test validating SummaryConcat expansion/runtime execution.
tests/ExpressiveSharp.Generator.Tests/ExpressiveGenerator/StringConcatenationTests.cs	New snapshot tests for string concatenation emission.
tests/ExpressiveSharp.Generator.Tests/ExpressiveGenerator/StringConcatenationTests.StringPlusString.verified.txt	New verified snapshot asserting Concat(string,string) emission.
tests/ExpressiveSharp.Generator.Tests/ExpressiveGenerator/StringConcatenationTests.IntPlusString.verified.txt	New verified snapshot asserting Concat(object,object) + Concat(string,string) emission.
tests/ExpressiveSharp.Generator.Tests/ExpressiveGenerator/StringConcatenationTests.SingleStringConcat.verified.txt	New verified snapshot asserting Concat(string,string) emission.
tests/ExpressiveSharp.Generator.Tests/ExpressiveGenerator/ExtensionMemberTests.ExtensionMemberWithMemberAccess.verified.txt	Updates snapshot to use Expression.Call(string.Concat, ...) instead of ExpressionType.Add.
tests/ExpressiveSharp.Generator.Tests/ExpressiveGenerator/ExtensionMemberTests.ExtensionMemberOnInterface.verified.txt	Updates snapshot to use Expression.Call(string.Concat, ...) instead of ExpressionType.Add.
tests/ExpressiveSharp.Generator.Tests/ExpressiveGenerator/ConstructorTests.ProjectableConstructor_WithSwitchExpression_AndExtraProperty.verified.txt	Updates snapshot string building to use string.Concat calls.
tests/ExpressiveSharp.Generator.Tests/ExpressiveGenerator/ConstructorTests.ProjectableConstructor_WithSwitchExpression.verified.txt	Updates snapshot string building to use string.Concat calls.
tests/ExpressiveSharp.Generator.Tests/ExpressiveGenerator/ConstructorTests.ProjectableConstructor_WithMultipleLocalVariables.verified.txt	Updates snapshot string building to use string.Concat calls.
tests/ExpressiveSharp.Generator.Tests/ExpressiveGenerator/ConstructorTests.ProjectableConstructor_WithLocalVariable.verified.txt	Updates snapshot string building to use string.Concat calls.
tests/ExpressiveSharp.Generator.Tests/ExpressiveGenerator/ConstructorTests.ProjectableConstructor_WithFullObject.verified.txt	Updates snapshot string building to use string.Concat calls (and renumbers cached MethodInfos).
tests/ExpressiveSharp.Generator.Tests/ExpressiveGenerator/ConstructorTests.ProjectableConstructor_ThisInitializer_WithIfElseInDelegated.verified.txt	Updates snapshot string building to use string.Concat calls.
tests/ExpressiveSharp.Generator.Tests/ExpressiveGenerator/ConstructorTests.ProjectableConstructor_ThisInitializer_WithBodyAfter.verified.txt	Updates snapshot string building to use string.Concat calls.
tests/ExpressiveSharp.Generator.Tests/ExpressiveGenerator/ConstructorTests.ProjectableConstructor_ThisInitializer_ChainedThisAndBase.verified.txt	Updates snapshot string building to use string.Concat calls.
tests/ExpressiveSharp.Generator.Tests/ExpressiveGenerator/ConstructorTests.ProjectableConstructor_ReferencingStaticConstMember.verified.txt	Updates snapshot string building to use string.Concat calls.
tests/ExpressiveSharp.Generator.Tests/ExpressiveGenerator/ConstructorTests.ProjectableConstructor_ReferencingPreviouslyAssignedProperty.verified.txt	Updates snapshot string building to use string.Concat calls.
tests/ExpressiveSharp.Generator.Tests/ExpressiveGenerator/ConstructorTests.ProjectableConstructor_ReferencingBasePropertyInDerivedBody.verified.txt	Updates snapshot string building to use string.Concat calls.

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