Cannot use functions in `op is`

[timotheeguerin]

says A value cannot be used as a type. , using an alias as intermediate fixes the issue

[github-actions]

🎯 Issue Triage

Using a function call directly in an op is statement causes a spurious "A value cannot be used as a type." error; using an alias as an intermediate workaround avoids the issue.

Category: Bug
Area: compiler:core

⚠️ No reproduction provided as text. A minimal playground repro would help diagnose this issue.

🔍 Analysis

• The error "A value cannot be used as a type." is being incorrectly raised when a function is used directly in an op is expression.
• Using an intermediate alias as a workaround suggests the compiler is mis-classifying the function return value as a value expression rather than a type in this syntactic position.
• Root cause is likely in how the compiler resolves and validates the type reference in the op is clause — it may not be applying the same type-vs-value disambiguation logic that it applies when resolving alias targets.
• Affected component: type checker / name resolution in @typespec/compiler.

📝 Suggested next steps

• Add a TypeSpec code snippet or playground link reproducing the error
• Investigate type resolution logic for op is in the compiler to ensure function calls returning types are treated as type expressions

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