[core] allow default values on non-optional properties

common/changes/@typespec/compiler/core-AllowRequiredDefault_2023-04-11-20-44.json

@@ -0,0 +1,10 @@
+{
+  "changes": [
+    {
+      "packageName": "@typespec/compiler",
+      "comment": "Allow use of defaults on non-optional properties.",
+      "type": "none"
+    }
+  ],
+  "packageName": "@typespec/compiler"
+}

docs/extending-typespec/emitters-basics.md

@@ -186,3 +186,22 @@ Since an emitter is a node library, you could use standard `fs` APIs to write fi
 Instead, use the compiler [`host` interface](#todo) to access the file system. The API is equivalent to the node API but works in a wider range of scenarios.
 
 In order to know where to emit files, the emitter context has a `emitterOutputDir` property that is automatically resolved using the `emitter-output-dir` built-in emitter options. This is set to `{cwd}/tsp-output/{emitter-name}` by default, but can be overridden by the user. Do not use the `compilerOptions.outputDir`
+
+## Handling Default Values
+
+Several TypeSpec types have a `default` property that can be used to specify a default value. For example, the following model has a default value of `true` for the `isActive` property:
+
+```tsp
+model User {
+  isActive?: boolean = true;
+}
+```
+
+These values can be accessed in the emitter using the `default` property on the `ModelProperty` type.
+
+```ts
+const modelProp: ModelProperty = ...;   // the isActive ModelProperty type
+const defaultValue = modelProp.default; // value: true
+```
+
+It is important that emitters handle default values in a consistent way. Default values SHOULD NOT be used as client-side default values. Instead, they should be used as a way to specify a default value for the server-side implementation. For example, if a model property has a default value of `true`, the server-side implementation should use that value if the client does not provide a value. Default values SHOULD be expressed in documentation to properly communicate the service-side default.

packages/compiler/core/messages.ts

@@ -117,12 +117,6 @@ const diagnostics = {
       topLevel: "Imports must be top-level and come prior to namespaces or other declarations.",
     },
   },
-  "default-optional": {
-    severity: "error",
-    messages: {
-      default: "Cannot use default with non optional properties",
-    },
-  },
   "token-expected": {
     severity: "error",
     messages: {

packages/compiler/core/parser.ts

@@ -853,9 +853,6 @@ function createParser(code: string | SourceFile, options: ParseOptions = {}): Pa
     const value = parseExpression();
 
     const hasDefault = parseOptional(Token.Equals);
-    if (hasDefault && !optional) {
-      error({ code: "default-optional" });
-    }
     const defaultValue = hasDefault ? parseExpression() : undefined;
     return {
       kind: SyntaxKind.ModelProperty,
@@ -2055,9 +2052,6 @@ function createParser(code: string | SourceFile, options: ParseOptions = {}): Pa
     const value = parseProjectionExpression();
 
     const hasDefault = parseOptional(Token.Equals);
-    if (hasDefault && !optional) {
-      error({ code: "default-optional" });
-    }
     const defaultValue = hasDefault ? parseProjectionExpression() : undefined;
     return {
       kind: SyntaxKind.ProjectionModelProperty,

packages/compiler/test/parser.test.ts

@@ -38,6 +38,10 @@ describe("compiler: parser", () => {
          prop2: string
        };`,
 
+      `model Car {
+         withDefaultButNotOptional: string = "foo"
+       }`,
+
       `model Car {
          optional?: number;
          withDefault?: string = "my-default";
@@ -110,10 +114,6 @@ describe("compiler: parser", () => {
       ["model Car is Foo extends Bar { }", [/'{' expected/]],
       ["model Car extends Bar is Foo { }", [/'{' expected/]],
       ["model Car { withDefaultMissing?: string =  }", [/Expression expected/]],
-      [
-        `model Car { withDefaultButNotOptional: string = "foo" }`,
-        [/Cannot use default with non optional properties/],
-      ],
       ["model", [/Identifier expected/]],
       ["model Car is Vehicle", [/';', or '{' expected/]],
       ["model Car;", [/'{', '=', 'extends', or 'is' expected/]],