#nullable enable

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using System.Linq;

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T Id<T>(T x) => x;

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T? Id2<T>(T? x) => x;

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var xs = new[] { (string?)"foo" };

var ys = xs.Select(Id);         // ❌ inferred type is IEnumerable<string>

string y = ys.Single();         // ❌ no warning CS8600: Converting [...] possible null value to non-nullable type.

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var zs = xs.Select(x => Id(x)); // ✔ inferred type is IEnumerable<string?>

string z = zs.Single();         // ✔ warning CS8600: Converting [...] possible null value to non-nullable type.

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var ys2 = xs.Select(Id2);       // ✔ inferred type is IEnumerable<string?>

string y2 = ys2.Single();       // ✔ warning CS8600: Converting [...] possible null value to non-nullable type.