fix(typescript-fetch): import FromJSON for discriminator mapped models

[kdelay]

Fixes #24654.

Mapped models are removed from tsImports (#15637 / #19195), so the discriminator import line in modelGeneric.mustache is their only import. It omits XFromJSON, which breaks when the parent also has a property typed as that child: deserialization emits XFromJSON(...) and tsc fails with TS2552.

Adding {{modelName}}FromJSON keeps that line's shape; it already imports XToJSON.

With the spec from the issue: TS2552 before, tsc --noEmit --strict clean after. Regenerated typescript-fetch samples. ./mvnw -B -pl modules/openapi-generator -am test: 4750 tests, 0 failures.

TypeScript (Fetch) technical committee: @leonyu

PR checklist

• Read the contribution guidelines.
• Built the project and regenerated the typescript-fetch samples.
• @mentioned the technical committee above.

---

Summary by cubic

Import FromJSON for discriminator-mapped models in the typescript-fetch generator to prevent TS2552 compile errors. Previously, discriminator imports omitted XFromJSON; now they include it, matching emitted XFromJSON(...) calls.

• Updated modelGeneric.mustache to add {{modelName}}FromJSON to the discriminator import list; regenerated typescript-fetch samples. No runtime impact; codegen-only change.

Rollout

• Regenerate clients with the typescript-fetch generator to pick up the fix. No other migration required.

^{Written for commit c254c23. Summary will update on new commits.}

[cubic-dev-ai]

1 issue found across 8 files

Prompt for AI agents (unresolved issues)

Check if these issues are valid — if so, understand the root cause of each and fix them. If appropriate, use sub-agents to investigate and fix each issue separately.


<file name="modules/openapi-generator/src/main/resources/typescript-fetch/modelGeneric.mustache">

<violation number="1" location="modules/openapi-generator/src/main/resources/typescript-fetch/modelGeneric.mustache:16">
P2: For discriminator parents whose mapped children are never referenced as property types (such as Animal.ts, where Cat/Dog only appear as subtypes), the newly added `FromJSON` import is unused. Consumers that compile generated code with `noUnusedLocals` enabled will now get a TS6133 error on the import, where previously `FromJSON` was not imported at all. This widens the pre-existing `ToJSON` unused-import situation to `FromJSON` across such models. Consider importing `FromJSON` only when the mapped model is also used as a property type, or documenting that consumers must not enable `noUnusedLocals`.</violation>
</file>

<sub>Reply with feedback, questions, or to request a fix.

Re-trigger cubic</sub>

[cubic-dev-ai]

P2: For discriminator parents whose mapped children are never referenced as property types (such as Animal.ts, where Cat/Dog only appear as subtypes), the newly added FromJSON import is unused. Consumers that compile generated code with noUnusedLocals enabled will now get a TS6133 error on the import, where previously FromJSON was not imported at all. This widens the pre-existing ToJSON unused-import situation to FromJSON across such models. Consider importing FromJSON only when the mapped model is also used as a property type, or documenting that consumers must not enable noUnusedLocals.

Prompt for AI agents

Check if this issue is valid — if so, understand the root cause and fix it. At modules/openapi-generator/src/main/resources/typescript-fetch/modelGeneric.mustache, line 16:

<comment>For discriminator parents whose mapped children are never referenced as property types (such as Animal.ts, where Cat/Dog only appear as subtypes), the newly added `FromJSON` import is unused. Consumers that compile generated code with `noUnusedLocals` enabled will now get a TS6133 error on the import, where previously `FromJSON` was not imported at all. This widens the pre-existing `ToJSON` unused-import situation to `FromJSON` across such models. Consider importing `FromJSON` only when the mapped model is also used as a property type, or documenting that consumers must not enable `noUnusedLocals`.</comment>

<file context>
@@ -13,7 +13,7 @@ import {
 {{#discriminator}}
 {{#discriminator.mappedModels}}
-import { type {{modelName}}, {{modelName}}FromJSONTyped, {{modelName}}ToJSON, {{modelName}}ToJSONTyped } from './{{modelName}}{{importFileExtension}}';
+import { type {{modelName}}, {{modelName}}FromJSON, {{modelName}}FromJSONTyped, {{modelName}}ToJSON, {{modelName}}ToJSONTyped } from './{{modelName}}{{importFileExtension}}';
 {{/discriminator.mappedModels}}
 {{/discriminator}}
</file context>