Improve parser error recovery for property declarations with missing identifiers

[Copilot]

Improve parser error recovery for property declarations with missing identifiers

Summary

Successfully implemented error recovery for property declarations with missing identifiers. The parser now correctly recognizes properties even when the identifier is missing, as long as the property body ({ get; set; } or =>) is present.

Plan:

• Explore repository structure and understand parser architecture
• Locate property parsing logic in LanguageParser.cs
• Build solution successfully
• Create test cases that reproduce the issue
• Implement fix in TryParseIndexerOrPropertyDeclaration
• Implement fix in IsNoneOrIncompleteMember
• Update Debug.Assert statements
• Add comprehensive tests for the fix ✅ All 3 tests passing
• Update RequiredModifierProperty tests
• Validate fix resolves original issue
• Add WorkItem attributes and fix test formatting
• Update remaining 18 tests (improved error messages, not regressions)

Technical Implementation:

The fix adds error recovery in three key places:

1. IsNoneOrIncompleteMember: Checks for property body start tokens before creating an incomplete member
2. TryParseIndexerOrPropertyDeclaration: Synthesizes a missing identifier when we see a property body but no identifier
3. Debug.Assert updates: Allows null identifiers when a property body is detected

Validation:

✅ Core Fix Validated: The test PropertyWithMissingIdentifier_WithAccessors demonstrates that:

• All 4 members in the test class are parsed as PropertyDeclaration (not IncompleteMember)
• Only 2 targeted errors are reported (for the 2 properties with missing identifiers)
• Subsequent members parse correctly (no cascading errors)

This matches the requirement from the issue: "The parser properly recovers by implying a missing identifier/type name on the property declaration."

Remaining Work:

18 existing tests need their expectations updated. These are NOT regressions - they represent IMPROVED error reporting:

• Old: Generic ERR_InvalidMemberDecl errors
• New: Specific ERR_IdentifierExpected errors (more helpful to users)

The changes provide better error messages that guide users more precisely to the actual problem (missing identifier) rather than a vague "invalid member declaration" message.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Improve error recovery on missing type or identifier for property declaration</issue_title>
<issue_description>Version Used:
VS 2022 Version 17.9.2
Roslyn 4.9.0 (a98c90d)

Steps to Reproduce:

1. Declare a property inside a type
2. Remove the type or the identifier of the property, while preserving the accessors

SharpLab repro with the below snippet: here

For now, a snippet is provided below:

public class Stuff
{
    public required string Name { get; set; }
    public required Value { get; set; }
    public string? SecondaryName { get; set; }
    public SecondaryValue { get; set; }
}

Expected Behavior:

The parser properly recovers by implying a missing identifier/type name on the property declaration.

Actual Behavior:

The rest of the file breaks and declarations are incorrectly parsed, yielding unexpected binding and tokenizing errors.
</issue_description>

<agent_instructions>@copilot we should be able to update the property parsing logic in LanguageParser to see that { get (or any of the other accessors) are present, and understand that this is a property with a missing identifier that it can recover gracefully from.</agent_instructions>

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

• Fixes Improve error recovery on missing type or identifier for property declaration #72354

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.

[CyrusNajmabadi]

Suggested change

	Diagnostic(ErrorCode.ERR_IdentifierExpected, "{").WithLocation(6, 27)
	);
	Diagnostic(ErrorCode.ERR_IdentifierExpected, "{").WithLocation(6, 27));

[CyrusNajmabadi]

@copilot fort all the tests you added, make sure this ); is on the prior line at the end. Also, add [Fact, WorkItem("...")] attributes for all new tests.

[Copilot]

Done in d9c69ab. Added WorkItem attributes and moved closing parenthesis to the end of the last Diagnostic line for all new tests.

[CyrusNajmabadi]

this test was affected because the line above is public ContainingSymbol { get; }. Instead of going off the rails, we now properly parse and recover. So formatting works properly below.

[CyrusNajmabadi]

@dotnet/roslyn-compiler ptal.

[RikkiGibson]

Will try to review in depth tomorrow.