Add BCP420

Author: mumian

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

@@ -399,7 +399,7 @@ If you need more information about a particular diagnostic code, select the **Fe
 | <a id='BCP417' />BCP417 | Error | The spread operator `{ellipsis}` cannot be used inside objects with property for-expressions.    |
 | <a id='BCP418' />BCP418 | Error | Extensions cannot be referenced here. Extensions can only be referenced by module extension configurations.   |
 | <a id='BCP419' />BCP419 | Error | Namespace name `{name}` cannot be used as an extension name.|
-| <a id='BCP420' />BCP420 | Error | The scope could not be resolved at compile time because the supplied expression is ambiguous or too complex. Scoping expressions must be reducible to a specific kind of scope without knowledge of parameter values. |
+| <a id='BCP420' />[BCP420](./diagnostics/bcp420.md) | Error | The scope could not be resolved at compile time because the supplied expression is ambiguous or too complex. Scoping expressions must be reducible to a specific kind of scope without knowledge of parameter values. |
 | <a id='BCP421' />BCP421 | Error | Module `{moduleName}` contains one or more secure outputs, which are not supported with `{LanguageConstants.TargetScopeKeyword}` set to `{LanguageConstants.TargetScopeTypeLocal}`.|
 | <a id='BCP422' />BCP422 | Error | A resource of type `{baseType}` may or may not exist when this function is called, which could cause the deployment to fail.|
 

articles/azure-resource-manager/bicep/diagnostics/bcp420.md

@@ -0,0 +1,42 @@
+---
+title: BCP420
+description: The scope couldn't be resolved at compile time because the supplied expression is ambiguous or too complex. Scoping expressions must be reducible to a specific kind of scope without knowledge of parameter values.
+ms.topic: reference
+ms.custom: devx-track-bicep
+ms.date: 06/25/2025
+---
+
+# Bicep diagnostic code - BCP420
+
+In Bicep, every resource or module must have a known deployment scope at compile time. The scope must be statically determinable. If the scope depends on a parameter, a variable, or an expression that can't be evaluated during compilation, Bicep throws BCP420.
+
+## Description
+
+The scope couldn't be resolved at compile time because the supplied expression is ambiguous or too complex. Scoping expressions must be reducible to a specific kind of scope without knowledge of parameter values.
+
+## Level
+
+Error
+
+## Examples
+
+The following code triggers BCP420 because the scope can't be determined at the compile time.
+
+```bicep
+param targetResourceGroupName string = 'my-target-rg'
+param storageAccountName string = 'mystorageacct'
+param location string = 'eastus'
+
+module storageModule './module.bicep' = {
+  name: 'deployStorage'
+  scope: empty(targetResourceGroupName) ? resourceGroup() : resourceGroup(targetResourceGroupName)
+  params: {
+    storageAccountName: storageAccountName
+    location: location
+  }
+}
+```
+
+## Next steps
+
+For more information about Bicep diagnostics, see [Bicep core diagnostics](../bicep-core-diagnostics.md).