List-patterns: report error when no suitable length member

src/Compilers/CSharp/Portable/Binder/Binder_Patterns.cs

@@ -289,7 +289,12 @@ private BoundListPattern BindListPattern(
             BoundListPatternReceiverPlaceholder? receiverPlaceholder;
             BoundListPatternIndexPlaceholder? argumentPlaceholder;
 
-            if (inputType.IsErrorType())
+            if (inputType.IsDynamic())
+            {
+                Error(diagnostics, ErrorCode.ERR_UnsupportedTypeForListPattern, node, inputType);
+            }
+
+            if (inputType.IsErrorType() || inputType.IsDynamic())
             {
                 hasErrors = true;
                 elementType = inputType;
@@ -337,13 +342,9 @@ private bool IsCountableAndIndexable(SyntaxNode node, TypeSymbol inputType, out
         private bool BindLengthAndIndexerForListPattern(SyntaxNode node, TypeSymbol inputType, uint inputValEscape, BindingDiagnosticBag diagnostics,
             out BoundExpression indexerAccess, out BoundExpression lengthAccess, out BoundListPatternReceiverPlaceholder? receiverPlaceholder, out BoundListPatternIndexPlaceholder argumentPlaceholder)
         {
-            bool hasErrors = false;
-            if (inputType.IsDynamic())
-            {
-                hasErrors |= true;
-                Error(diagnostics, ErrorCode.ERR_UnsupportedTypeForListPattern, node, inputType);
-            }
+            Debug.Assert(!inputType.IsDynamic());
 
+            bool hasErrors = false;
             receiverPlaceholder = new BoundListPatternReceiverPlaceholder(node, GetValEscape(inputType, inputValEscape), inputType) { WasCompilerGenerated = true };
             if (inputType.IsSZArray())
             {
@@ -359,7 +360,11 @@ private bool BindLengthAndIndexerForListPattern(SyntaxNode node, TypeSymbol inpu
             }
             else
             {
-                hasErrors |= !TryBindLengthOrCount(node, receiverPlaceholder, out lengthAccess, diagnostics);
+                if (!TryBindLengthOrCount(node, receiverPlaceholder, out lengthAccess, diagnostics))
+                {
+                    hasErrors = true;
+                    Error(diagnostics, ErrorCode.ERR_ListPatternRequiresLength, node, inputType);
+                }
             }
 
             var analyzedArguments = AnalyzedArguments.GetInstance();

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -6760,6 +6760,9 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="ERR_UnsupportedTypeForListPattern" xml:space="preserve">
     <value>List patterns may not be used for a value of type '{0}'.</value>
   </data>
+  <data name="ERR_ListPatternRequiresLength" xml:space="preserve">
+    <value>List patterns may not be used for a value of type '{0}'. No suitable 'Length' or 'Count' property was found.</value>
+  </data>
   <data name="ERR_UnsupportedTypeForSlicePattern" xml:space="preserve">
     <value>Slice patterns may not be used for a value of type '{0}'.</value>
   </data>

src/Compilers/CSharp/Portable/Errors/ErrorCode.cs

@@ -2020,6 +2020,7 @@ internal enum ErrorCode
         ERR_StructHasInitializersAndNoDeclaredConstructor = 8983,
         ERR_EncUpdateFailedDelegateTypeChanged = 8984,
 
+        ERR_ListPatternRequiresLength = 8985,
         ERR_DiscardCannotBeNullChecked = 8990,
         ERR_MustNullCheckInImplementation = 8991,
         ERR_NonNullableValueTypeIsNullChecked = 8992,

src/Compilers/CSharp/Test/IOperation/IOperation/IOperationTests_IIsPatternExpression.cs

@@ -2319,6 +2319,9 @@ void M()
 ";
             var expectedDiagnostics = new[]
             {
+                // (8,31): error CS8985: List patterns may not be used for a value of type 'X'. No suitable 'Length' or 'Count' property was found.
+                //         _ = /*<bind>*/this is []/*</bind>*/;
+                Diagnostic(ErrorCode.ERR_ListPatternRequiresLength, "[]").WithArguments("X").WithLocation(8, 31),
                 // (8,31): error CS0518: Predefined type 'System.Index' is not defined or imported
                 //         _ = /*<bind>*/this is []/*</bind>*/;
                 Diagnostic(ErrorCode.ERR_PredefinedTypeNotFound, "[]").WithArguments("System.Index").WithLocation(8, 31)