Two different metadata files with the same name in different folders: the generator silently uses whichever comes first, with no diagnostic

[matt-edmondson]

What's wrong

GeneratorBase (SourceGeneratorToolkit/GeneratorBase.cs ~lines 70–76) groups the AdditionalTexts by file name and keeps the first non-empty one:

// A duplicate name means the same metadata reached the compilation twice; the first with
// content wins.
.GroupBy(file => file.FileName, StringComparer.Ordinal)
.ToDictionary(g => g.Key, g => g.FirstOrDefault(file => file.Text.Length > 0) ?? g.First(), ...)

The comment assumes both copies hold the same metadata. Nothing checks that. When two files share a name but have different content, one is dropped silently.

Repro (verified by driving CSharpGeneratorDriver directly against current main)

Give a generator that wants things.json two AdditionalTexts:

• /a/Legacy/things.json defining Old
• /a/Metadata/things.json defining New

The run reports 0 diagnostics and emits public class Old. Which file wins depends on item order. The easy ways to hit this are <AdditionalFiles Include="**/*.json" /> and a stale copy left in another folder.

GeneratorTestHarness can't show this, because it keys its input files by name.

Why it matters

The package's own rule, stated a few lines further down, is that producing no output or the wrong output with no explanation must always be reported. This path generates the wrong code with no explanation.

Suggested fix

When a group holds more than one non-empty file and their texts differ, report a diagnostic that names both paths. That could be a new overridable descriptor such as MetadataFileDuplicated, or MetadataParseFailed with a "supplied more than once: , " reason. Identical duplicates can stay silent, as they are today.

Acceptance criteria

• Two same-named metadata files with different content produce a diagnostic naming both paths.
• Two same-named files with identical content still produce none.

[matt-edmondson]

Triage

• Category: Bug (a missing diagnostic that leads to wrong generated code)
• Priority: Medium. It needs two differing same-named files, but **/*.json globs make that easy, and the result is wrong code with no explanation. That breaks the package's own "always report" rule.
• Area / suggested assignment: the AdditionalTexts grouping in SourceGeneratorToolkit/GeneratorBase.cs, plus a new or reused diagnostic descriptor. GeneratorTestHarness would need to accept duplicate names (for example, keyed by path) to test this.
• Duplicates: none. It is closely related to A misspelled or duplicated key in metadata JSON is silently ignored, so the generator emits wrong output with no diagnostic #31 (misspelled or duplicated JSON keys silently ignored) and Identical metadata in a new AdditionalText instance re-runs the whole generator, because MetadataFile has no value equality #27 (the equality and caching of MetadataFile). All three touch the same metadata-ingest path, so it is worth doing them together.
• In progress: no. The only open PR is a Dependabot bump (Bump the ktsu group with 10 updates #29).

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