Fix ICE when interpolating a function call in a template declaration

[timotheeguerin]

Fixes #11054

Problem

Compiling a string template that interpolates a function call referencing a template parameter on a template declaration crashed with an internal compiler error:

extern fn fn(type: unknown): valueof string;

@doc("${fn(T)}")
model Crud<T extends Reflection.Model> {}

Error: Expected type.
    at checkStringTemplateExpresion (packages/compiler/src/core/checker.ts)

Root cause

In a template declaration the function call can't be evaluated yet, so checkFunctionCall → getDefaultFunctionResult returns null for a value-returning function (the same behavior already used for a direct decorator arg like @doc(fn(T))). null is the value-world equivalent of errorType.

checkStringTemplateExpresion didn't handle a null span: both hasValue/hasType stayed false, execution fell into the type branch, and compilerAssert(typeOrValue !== null && !isValue(typeOrValue), "Expected type.") fired.

Fix

If any span resolves to null, the whole string template resolves to null — propagating the error sentinel up exactly like checkArrayValue/checkObjectValue do for value aggregates, rather than fabricating a partial string. The value is re-evaluated against the real argument when the template is instantiated. This also let the now-redundant per-branch null checks be removed.

Tests

Added regression tests in string-template.test.ts:

• A function call interpolated in a string template on a template declaration no longer crashes and reports no diagnostics.
• The instantiation path still resolves the function call (Crud<Foo> produces the expected doc value).

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/@typespec/compiler@11056

commit: 2260527

[github-actions]

All changed packages have been documented.

• ✅ @typespec/compiler

Show changes

@typespec/compiler - fix ✏️

Fix Expected type. internal compiler error when a string template interpolates a function call that references a template parameter on a template declaration (e.g. @doc("${myFn(T)}") model Crud<T extends Reflection.Model> {}). The deferred function call now defers the whole template, which is evaluated at instantiation.

[azure-sdk-automation]

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