Collapse target union types before narrowing (#10987)

Resolves #10098

When narrowing types, we should collapse unions as an early first step.
This is primarily done to enhance constraint checks and error messaging

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src/Bicep.Core.IntegrationTests/ScenarioTests.cs

@@ -3308,7 +3308,7 @@ public void Test_Issue_3356_Warn_On_Bad_Type_Definitions()
             ("BCP081", DiagnosticLevel.Warning, "Resource type \"Microsoft.Storage/storageAccounts@2021-09-00\" does not have types available."),
             ("BCP036", DiagnosticLevel.Warning, "The property \"name\" expected a value of type \"string\" but the provided value is of type \"123\". If this is an inaccuracy in the documentation, please report it to the Bicep Team."),
             ("BCP036", DiagnosticLevel.Warning, "The property \"capacity\" expected a value of type \"int\" but the provided value is of type \"'1'\". If this is an inaccuracy in the documentation, please report it to the Bicep Team."),
-            ("BCP036", DiagnosticLevel.Warning, "The property \"type\" expected a value of type \"'ArcZone' | 'CustomLocation' | 'EdgeZone' | 'NotSpecified' | string\" but the provided value is of type \"1\". If this is an inaccuracy in the documentation, please report it to the Bicep Team."),
+            ("BCP036", DiagnosticLevel.Warning, "The property \"type\" expected a value of type \"string\" but the provided value is of type \"1\". If this is an inaccuracy in the documentation, please report it to the Bicep Team."),
             ("BCP036", DiagnosticLevel.Warning, "The property \"capacity\" expected a value of type \"int\" but the provided value is of type \"'2'\". If this is an inaccuracy in the documentation, please report it to the Bicep Team."),
             ("BCP036", DiagnosticLevel.Warning, "The property \"tenantId\" expected a value of type \"string\" but the provided value is of type \"3\". If this is an inaccuracy in the documentation, please report it to the Bicep Team."),
             ("BCP036", DiagnosticLevel.Warning, "The property \"clientId\" expected a value of type \"string\" but the provided value is of type \"1\". If this is an inaccuracy in the documentation, please report it to the Bicep Team."),
@@ -4790,4 +4790,31 @@ param resourceGroups resourceGroup[]
             ("BCP035", DiagnosticLevel.Error, "The specified \"object\" declaration is missing the following required properties: \"bar\", \"foo\"."),
         });
     }
+
+    // https://github.com/Azure/bicep/issues/10098
+    [TestMethod]
+    public void Test_Issue10098()
+    {
+        var result = CompilationHelper.Compile(Services.WithFeatureOverrides(new(UserDefinedTypesEnabled: true)),
+("mod.bicep", @"
+@allowed([0, 1, 2, 3 ])
+param availabilityZone int = 0
+
+param availabilityZoneUnion 0 | 1 | 2 | 3 = 0
+"),
+("main.bicep", @"
+@minValue(0)
+param count int
+
+module mod 'mod.bicep' = [for i in range(0, count): {
+  name: 'mod${i}'
+  params: {
+    availabilityZone: i % 3 + 1
+    availabilityZoneUnion: i % 3 + 1
+  }
+}]
+"));
+
+        result.ExcludingLinterDiagnostics().Should().NotHaveAnyDiagnostics();
+    }
 }

src/Bicep.Core.Samples/Files/IntermediaryVariables_LF/main.diagnostics.bicep

@@ -17,7 +17,7 @@ resource vm 'Microsoft.Compute/virtualMachines@2020-12-01' = {
 //@[12:21) [no-hardcoded-location (Warning)] A resource location should not use a hard-coded string or variable value. Please use a parameter value, an expression, or the string 'global'. Found: 'West US' (CodeDescription: bicep core(https://aka.ms/bicep/linter/no-hardcoded-location)) |'West US'|
   properties: vmProperties
 //@[14:26) [BCP036 (Warning)] The property "enabled" expected a value of type "bool | null" but the provided value in source declaration "vmProperties" is of type "123". If this is an inaccuracy in the documentation, please report it to the Bicep Team. (CodeDescription: bicep(https://aka.ms/bicep-type-issues)) |vmProperties|
-//@[14:26) [BCP036 (Warning)] The property "evictionPolicy" expected a value of type "'Deallocate' | 'Delete' | null | string" but the provided value in source declaration "vmProperties" is of type "true". If this is an inaccuracy in the documentation, please report it to the Bicep Team. (CodeDescription: bicep(https://aka.ms/bicep-type-issues)) |vmProperties|
+//@[14:26) [BCP036 (Warning)] The property "evictionPolicy" expected a value of type "null | string" but the provided value in source declaration "vmProperties" is of type "true". If this is an inaccuracy in the documentation, please report it to the Bicep Team. (CodeDescription: bicep(https://aka.ms/bicep-type-issues)) |vmProperties|
 //@[14:26) [BCP036 (Warning)] The property "storageUri" expected a value of type "null | string" but the provided value in source declaration "vmProperties" is of type "true". If this is an inaccuracy in the documentation, please report it to the Bicep Team. (CodeDescription: bicep(https://aka.ms/bicep-type-issues)) |vmProperties|
 //@[14:26) [BCP037 (Warning)] The property "unknownProp" from source declaration "vmProperties" is not allowed on objects of type "BootDiagnostics". No other properties are allowed. If this is an inaccuracy in the documentation, please report it to the Bicep Team. (CodeDescription: bicep(https://aka.ms/bicep-type-issues)) |vmProperties|
 }

src/Bicep.Core/TypeSystem/TypeValidator.cs

@@ -270,6 +270,9 @@ private TypeSymbol NarrowType(TypeValidatorConfig config, SyntaxBase expression,
 
                         return new TypedArrayType(narrowedBody, TypeSymbolValidationFlags.Default);
                     }
+                case UnionType when TypeCollapser.TryCollapse(targetType) is TypeSymbol collapsed:
+                    targetType = collapsed;
+                    break;
             }
 
             // basic assignability check