Fix CS0419 to display fully qualified type names for ambiguous cref symbols

[Copilot]

Plan for fixing CS0419 to include fully qualified references

• Understand the issue: CS0419 warning message should show fully qualified names like CS0104 does
• Locate the code that generates the warning in Binder_Crefs.cs
• Identify that CS0104 uses SymbolDisplayFormat.CSharpErrorMessageFormat for displaying symbols
• Create a test case that reproduces the issue from the problem statement
• Modify the code to use FormattedSymbol with CSharpErrorMessageFormat in both locations
• Add test for the new behavior with types in different namespaces
• Build and test the changes - all tests pass (225 CrefTests, all CS0419 tests pass)
• Run code review and security checks - no issues found
• Fix WorkItem attribute to reference correct issue Include fully qualified references in CS0419 to make it less confusing #4031
• Make test classes non-public to simplify test expectations
• Add test for ambiguous method references with parameters (covers PerformCrefOverloadResolution path)

Summary

Successfully fixed CS0419 to display fully qualified type names in ambiguous reference warnings, matching the behavior of CS0104. The warning message now clearly shows the namespace qualification of ambiguous types and methods, making it obvious where the conflicts are coming from.

Changes Made

1. Binder_Crefs.cs: Modified two locations where WRN_AmbiguousXMLReference is generated:

   • Line 911: ProcessParameterlessCrefMemberLookupResults - tested by AmbiguousReferenceInDifferentNamespaces
   • Line 1069: PerformCrefOverloadResolution - tested by AmbiguousReferenceInDifferentNamespaces_WithParameters
   • Both now wrap symbols with FormattedSymbol using SymbolDisplayFormat.CSharpErrorMessageFormat

2. CrefTests.cs: Added test cases that verify the new behavior:

   • AmbiguousReferenceInDifferentNamespaces: Tests ambiguous types without parameter lists
   • AmbiguousReferenceInDifferentNamespaces_WithParameters: Tests ambiguous methods with parameter lists
   • Both tests verify fully qualified names appear in diagnostics (WorkItem Include fully qualified references in CS0419 to make it less confusing #4031)

Example

Before:

CS0419  Ambiguous reference in cref attribute: 'TypeA'. Assuming 'TypeA', but could have also matched other overloads including 'TypeA'.

After:

CS0419  Ambiguous reference in cref attribute: 'TypeA'. Assuming 'System.Goo.TypeA', but could have also matched other overloads including 'System.TypeA'.

• Fixes Include fully qualified references in CS0419 to make it less confusing #4031

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Include fully qualified references in CS0419 to make it less confusing</issue_title>
<issue_description>Given the following:

namespace System
{
    public class TypeA
    {
    }
}

namespace System.Foo
{
    public class TypeA
    {
    }
}

namespace A
{
    using System;
    using System.Foo;

    /// <summary>
    ///     <see cref="TypeA"/>
    /// </summary>
    public class Bar
    {
    }
}

You get the following:

CS0419  Ambiguous reference in cref attribute: 'TypeA'. Assuming 'TypeA', but could have also matched other overloads including 'TypeA'.

This is confusing, it should fully qualify the type to make it obvious where the two references are coming from, similar to CS0104:

CS0104  'TypeA' is an ambiguous reference between 'System.Foo.TypeA' and 'System.TypeA'

</issue_description>

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

• Fixes Include fully qualified references in CS0419 to make it less confusing #4031

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[CyrusNajmabadi]

@copilot should be #4031

[Copilot]

Fixed in eaef250

[CyrusNajmabadi]

@copilot makes these non-public, so you don't have to have the warning diagnostics about missing xml doc comments.

[Copilot]

Fixed in 3effaa8

[CyrusNajmabadi]

@RikkiGibson ptal.