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invalid-alternate-type

Id
@azure-tools/typespec-client-generator-core/invalid-alternate-type

Severity: error

This diagnostic is issued when @alternateType is applied to a scalar source type with a non-scalar alternate type.

Impact

Section titled “Impact”
  • Area: SDK type substitution. Blocks an invalid @alternateType replacement that would change a scalar into a non-scalar SDK type.
  • Not affected: The source scalar and its wire encoding remain unchanged.

❌ Incorrect Usage

Section titled “❌ Incorrect Usage”
scalar storageDateTime extends utcDateTime;
model DateWrapper {
  value: string;
}
@@alternateType(storageDateTime, DateWrapper); // scalar source cannot use a model alternate type

Diagnostic Message

Section titled “Diagnostic Message”

TCGC reports:

Invalid alternate type. If the source type is Scalar, the alternate type must also be Scalar. Found alternate type kind: 'Model'

✅ How to Fix

Section titled “✅ How to Fix”

Use a scalar alternate type when the source is a scalar, or apply @alternateType to a non-scalar source when replacing it with a non-scalar shape:

scalar clientDateTime extends string;
@@alternateType(storageDateTime, clientDateTime);