Fix type checking for sys.allowed

src/Bicep.Core.IntegrationTests/ScenarioTests.cs

@@ -1880,4 +1880,21 @@ public void Test_Issue2262()
             codeReplacement.Span.Should().Be(new TextSpan(212, 15));
             codeReplacement.Text.Should().Be("partitionScheme");
         }
+
+        [TestMethod]
+        // https://github.com/Azure/bicep/issues/2484
+        public void Test_Issue2484()
+        {
+            var result = CompilationHelper.Compile(@"
+@sys.allowed([
+  'apple'
+  'banana'
+]) 
+param foo string = 'peach'
+");
+
+            result.Should().HaveDiagnostics(new[] {
+                ("BCP027", DiagnosticLevel.Error, "The parameter expects a default value of type \"'apple' | 'banana'\" but provided value is of type \"'peach'\"."),
+            });
+        }
     } }

src/Bicep.Core/TypeSystem/TypeAssignmentVisitor.cs

@@ -317,6 +317,20 @@ public override void VisitParameterDeclarationSyntax(ParameterDeclarationSyntax
                  */
                 var allowedDecoratorSyntax = syntax.Decorators.FirstOrDefault(decoratorSyntax =>
                 {
+                    if (decoratorSyntax.Expression is InstanceFunctionCallSyntax { BaseExpression: VariableAccessSyntax namespaceSyntax, Name: IdentifierSyntax nameSyntax } &&
+                        namespaceSyntax.Name.IdentifierName == "sys" &&
+                        nameSyntax.IdentifierName == "allowed")
+                    {
+                        /*
+                         * This is a workaround to unambiguously get the syntax for "@sys.allowed(...)".
+                         * We cannot get it by resolving the function symbol, since currently the binder
+                         * does not bind an InstanceFunctionCallSyntax to a symbol. This does not affect
+                         * decorator emitting though, because in the emitter we call SemanticModel.GetSymbolInfo()
+                         * which adds special handling for InstanceFunctionCallSyntax.
+                         */
+                        return true;
+                    }
+
                     if (this.binder.GetSymbolInfo(decoratorSyntax.Expression) is FunctionSymbol functionSymbol)
                     {
                         var argumentTypes = this.GetRecoveredArgumentTypes(decoratorSyntax.Arguments).ToArray();