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

.chronus/changes/decl-expr-declaration-expressions-2026-4-18-0-0-0.md

@@ -0,0 +1,25 @@
+---
+changeKind: feature
+packages:
+  - "@typespec/compiler"
+---
+
+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 `""`).
+
+They can be used anywhere an expression is expected, including aliases, model properties, decorator arguments, template arguments, function/call arguments, and tuples.
+
+```tsp
+alias Foo = enum {
+  a,
+  b,
+};
+
+model Bar {
+  status: enum { active, inactive };
+  unit: scalar extends string;
+  inner: model Inner { x: string };
+}
+
+@Versioning.versioned(enum Versions { v1, v2 })
+namespace MyService;
+```

.chronus/changes/decl-expr-decorators-2026-4-18-0-0-2.md

@@ -0,0 +1,16 @@
+---
+changeKind: feature
+packages:
+  - "@typespec/compiler"
+---
+
+Allow decorators to be applied to `model`, `enum`, `union`, and `scalar` declarations used in expression position. Inline decorators can be applied directly, and augment decorators (`@@`) can target them through a navigation reference (such as `::type`).
+
+```tsp
+model Foo {
+  status: @doc("the current status") enum { active, inactive };
+  inner: @doc("nested model") model Inner { x: string };
+}
+
+@@doc(Foo.status::type, "the current status");
+```

.chronus/changes/decl-expr-doc-comments-2026-4-18-0-0-3.md

@@ -0,0 +1,13 @@
+---
+changeKind: feature
+packages:
+  - "@typespec/compiler"
+---
+
+Allow a doc comment to be applied inline to a `model`, `enum`, `union`, or `scalar` declaration expression, just like an inline `@doc` decorator.
+
+```tsp
+model Foo {
+  status: /** the current status */ enum { active, inactive };
+}
+```

.chronus/changes/decl-expr-formatting-2026-4-18-0-0-4.md

@@ -0,0 +1,19 @@
+---
+changeKind: feature
+packages:
+  - "@typespec/compiler"
+---
+
+Improve formatting of declaration expressions (`model`, `enum`, `union`, and `scalar` used in expression position) that carry doc comments and/or decorators. When the inline form would exceed the print width, the doc comments and decorators are now each placed on their own line and the whole block is indented one level instead of overflowing.
+
+```tsp
+model Foo {
+  status:
+    @summary("a fairly long summary text here")
+    @example("some-default-example-value")
+    enum {
+      active,
+      inactive,
+    };
+}
+```

.chronus/changes/decl-expr-json-schema-inline-2026-4-18-0-0-1.md

@@ -0,0 +1,15 @@
+---
+changeKind: feature
+packages:
+  - "@typespec/json-schema"
+---
+
+Support `model`, `enum`, `union`, and `scalar` declarations used in expression position. Anonymous declaration expressions are inlined, while named ones are hoisted into their own schema.
+
+```tsp
+model Foo {
+  status: enum { active, inactive }; // inlined
+  unit: scalar extends string; // inlined
+  inner: model Inner { x: string }; // hoisted as `Inner.json`
+}
+```

.chronus/changes/decl-expr-openapi-inline-2026-4-18-0-0-1.md

@@ -0,0 +1,16 @@
+---
+changeKind: feature
+packages:
+  - "@typespec/openapi"
+  - "@typespec/openapi3"
+---
+
+Support `model`, `enum`, `union`, and `scalar` declarations used in expression position. Anonymous declaration expressions are inlined, while named ones are hoisted into a referenced component.
+
+```tsp
+model Foo {
+  status: enum { active, inactive }; // inlined
+  unit: scalar extends string; // inlined
+  inner: model Inner { x: string }; // hoisted as component `Inner`
+}
+```

.chronus/changes/decl-expr-typekit-enum-2026-4-18-0-0-1.md

@@ -0,0 +1,7 @@
+---
+changeKind: feature
+packages:
+  - "@typespec/compiler"
+---
+
+`$.enum.create` now produces an enum expression (`expression: true`) when given an empty `name`, mirroring `$.model.create`.

.chronus/changes/decl-expr-versioning-validation-2026-4-18-0-0-1.md

@@ -0,0 +1,7 @@
+---
+changeKind: feature
+packages:
+  - "@typespec/versioning"
+---
+
+Validate the variants of a keyword-form union expression (`union { ... }`) used in expression position like the variants of a named union, so versioning incompatibilities on decorated variants are reported.

.chronus/changes/html-program-viewer-expression-2026-4-18-0-0-0.md

@@ -0,0 +1,7 @@
+---
+changeKind: feature
+packages:
+  - "@typespec/html-program-viewer"
+---
+
+Display the new `expression` property on `Model`, `Enum`, and `Scalar` types in the program viewer.