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

.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/declarations-as-expressions-2026-4-18-0-0-0.md

@@ -0,0 +1,20 @@
+---
+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 `""`).
+
+```tsp
+alias Foo = enum {
+  a,
+  b,
+};
+
+model Bar {
+  status: enum { active, inactive };
+  unit: scalar extends string;
+  inner: model Inner { x: string };
+}
+```

.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.