Merge pull request #309112 from mumian/1205-bcp293

Document BCP293

Author: denrea

articles/azure-resource-manager/bicep/bicep-core-diagnostics.md

@@ -285,7 +285,7 @@ If you need more information about a particular diagnostic code, select the **Fe
 | <a id='BCP290' />[BCP290](./diagnostics/bcp290.md) | Error | Expected a parameter or type declaration after the decorator. |
 | <a id='BCP291' />BCP291 | Error | Expected a parameter or output declaration after the decorator. |
 | <a id='BCP292' />[BCP292](./diagnostics/bcp292.md) | Error | Expected a parameter, output, or type declaration after the decorator. |
-| <a id='BCP293' />BCP293 | Error | All members of a union type declaration must be literal values. |
+| <a id='BCP293' />[BCP293](./diagnostics/bcp293.md) | Error | All members of a union type declaration must be literal values. |
 | <a id='BCP294' />[BCP294](./diagnostics/bcp294.md) | Error | Type unions must be reducible to a single ARM type (such as `string`, `int`, or `bool`).      |
 | <a id='BCP295' />BCP295 | Error | The `{decoratorName}` decorator may not be used on targets of a union or literal type. The allowed values for this parameter or type definition will be derived from the union or literal type automatically. |
 | <a id='BCP296' />BCP296 | Error | Property names on types must be compile-time constant values. |

articles/azure-resource-manager/bicep/diagnostics/bcp293.md

@@ -0,0 +1,59 @@
+---
+title: BCP293
+description: All members of a union type declaration must be literal values.
+ms.topic: reference
+ms.custom: devx-track-bicep
+ms.date: 12/05/2025
+---
+
+# Bicep diagnostic code - BCP293
+
+This diagnostic occurs when you declare a union type with members that aren't literal values.
+
+## Description
+
+All members of a union type declaration must be literal values.
+
+## Level
+
+Error
+
+## Solution
+
+Ensure that all members of a union type declaration are literal values. For more information, see [User-defined data types](../user-defined-data-types.md).
+
+## Examples
+
+The following example raises the diagnostic because the intent behind the following code is to restrict the input to specific environments ('dev', 'test', 'prod') while also trying to allow any other string value. Bicep's type system doesn't permit this mix in a union.
+
+```bicep
+type EnvironmentType = 'dev' | 'test' | 'prod' | string 
+```
+
+If the goal is to strictly enforce the three allowed values and fail compilation otherwise, remove the generic string type.
+
+```bicep
+type EnvironmentType = 'dev' | 'test' | 'prod'
+```
+
+If the goal is to allow any string value and the predefined list is just a suggestion, use the generic `string` type directly and remove the union altogether.
+
+```bicep
+type EnvironmentType = string
+```
+
+The following example is another example where mixing literals with objects containing generic primitives isn't allowed:
+
+```bicep
+type oneOfSeveralObjects = {foo: 'bar', bar: string} | {fizz: 'buzz'} | {snap: 'crackle'}
+```
+
+To fix the problem, all members must be of the same type category. Since the intent here is a union of complex types, the simplest fix is to ensure all properties that are intended to be constants are defined as string literals throughout the union.
+
+```bicep
+type oneOfSeveralObjects = {foo: 'bar', bar: 'baz'} | {fizz: 'buzz'} | {snap: 'crackle'}
+```
+
+## Next steps
+
+For more information about Bicep diagnostics, see [Bicep core diagnostics](../bicep-core-diagnostics.md).

articles/azure-resource-manager/bicep/toc.yml

@@ -739,6 +739,8 @@ items:
             href: diagnostics/bcp290.md
           - name: BCP292
             href: diagnostics/bcp292.md
+          - name: BCP293
+            href: diagnostics/bcp293.md
           - name: BCP294
             href: diagnostics/bcp294.md
           - name: BCP302

articles/azure-resource-manager/bicep/user-defined-data-types.md

@@ -143,7 +143,13 @@ The valid type expressions include:
 - Unions can include any number of literal-typed expressions. Union types are translated into the [allowed-value constraint](./parameters.md#use-decorators) in Bicep, so only literals are permitted as members.
 
     ```bicep
-    type oneOfSeveralObjects = {foo: 'bar'} | {fizz: 'buzz'} | {snap: 'crackle'}
+    type oneOfSeveralObjects = {
+      foo: 'bar'
+    } | {
+      fizz: 'buzz'
+    } | {
+      snap: 'crackle'
+    }
     type mixedTypeArray = ('fizz' | 42 | {an: 'object'} | null)[]
     ```