feat(compiler): allow declarations to be used as expressions

[timotheeguerin]

Summary

Allow model, enum, union, and scalar declarations to be used as expressions (e.g. the right-hand side of an alias, property types, return types, union variants).

In expression position these declarations:

• are anonymous — their name is ""
• produce a type with expression: true (so the type graph explicitly records that it came from an expression)
• are not registered in the enclosing namespace

alias Foo = enum {
  a,
  b,
};

model Bar {
  status: enum { active, inactive };
  unit: scalar extends string;
  inner: model { x: string };
}

Named forms in expression position are also allowed (e.g. nested: model Inner { x: string }), and declarations can be nested.

Approach

Rather than introducing new *Expression AST node kinds, the existing statement node kinds are reused with an optional id (OptionallyNamedDeclarationNode), and these kinds are added to the Expression union. Whether a declaration is in statement vs expression position is determined by its parent node kind.

• types.ts — OptionallyNamedDeclarationNode (optional id); expression: boolean on Model/Enum/Scalar (Union already had it); the 4 statement kinds added to the Expression union.
• parser.ts — optional-id parsing on the 4 declaration parsers; parsePrimaryExpression dispatch for the model/enum/union/scalar keywords; a scalar in expression position no longer consumes the alias trailing ;.
• binder.ts — anonymous ("-" symbol) vs named binding depending on position.
• checker.ts — derive name/expression; skip namespace registration for expression-form types; route expression decls through the namespace walk-up in getParentNamespaceType; new templated-declaration-in-expression diagnostic.
• formatter — optional-id printing; avoids a double ; for anonymous scalars.
• server/inspector — null-safe handling of the now-optional id.

Diagnostics

Template parameters on a declaration in expression position are rejected (templated-declaration-in-expression) since an anonymous declaration cannot be referenced or instantiated.

Tests

• test/checker/declaration-expressions.test.ts (14 tests)
• test/parser.test.ts — new "declaration expressions" block (11 cases)
• test/formatter/formatter.test.ts — new "declaration expressions" block (5 cases)

Full compiler suite: 3982 passed / 6 skipped. tsc --noEmit, pnpm format, and pnpm lint are all clean.

Follow-ups (not in this PR)

• Deeper TextMate grammar rules specific to keyword-declarations in expression position.
• Policy for a name on a pure-expression declaration (currently kept on Type.name but not referenceable).
• Validate downstream emitter packages against the new Expression union members.

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/@typespec/compiler@11019

npm i https://pkg.pr.new/@typespec/html-program-viewer@11019

commit: 44e5e41

[github-actions]

All changed packages have been documented.

• ✅ @typespec/compiler
• ✅ @typespec/html-program-viewer

Show changes

@typespec/compiler - feature ✏️

Allow model, enum, union, and scalar declarations to be used as expressions. A declaration used in expression position has its corresponding type marked with expression: true and is not registered in the enclosing namespace. It may be named or anonymous (in which case its name is "").,> ,> tsp,> alias Foo = enum {,> a,,> b,,> };,> ,> model Bar {,> status: enum { active, inactive };,> unit: scalar extends string;,> inner: model Inner { x: string };,> },>

@typespec/html-program-viewer - feature ✏️

Display the new expression property on Model, Enum, and Scalar types in the program viewer.

[azure-sdk-automation]

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