dotnet · jjonescz · Jul 19, 2023 · Jul 14, 2023 · Jul 17, 2023 · Jul 18, 2023
@@ -4094,6 +4094,12 @@ You should consider suppressing the warning only if you're sure that you don't w
   <data name="WRN_DefaultValueForUnconsumedLocation_Title" xml:space="preserve">
     <value>The default value specified will have no effect because it applies to a member that is used in contexts that do not allow optional arguments</value>
   </data>
+  <data name="WRN_RefReadonlyParameterDefaultValue" xml:space="preserve">
+    <value>A default value is specified for 'ref readonly' parameter '{0}', but 'ref readonly' should be used only for references. Consider declaring the parameter as 'in'.</value>
+  </data>
+  <data name="WRN_RefReadonlyParameterDefaultValue_Title" xml:space="preserve">
+    <value>A default value is specified for 'ref readonly' parameter, but 'ref readonly' should be used only for references. Consider declaring the parameter as 'in'.</value>
+  </data>
   <data name="ERR_PublicKeyFileFailure" xml:space="preserve">
     <value>Error signing output with public key from file '{0}' -- {1}</value>
   </data>

@@ -2249,6 +2249,7 @@ internal enum ErrorCode
         WRN_OverridingDifferentRefness = 9507,
         WRN_HidingDifferentRefness = 9508,
         ERR_OutAttrOnRefReadonlyParam = 9520,
+        WRN_RefReadonlyParameterDefaultValue = 9521,
 
         #endregion
 

@@ -542,6 +542,7 @@ internal static int GetWarningLevel(ErrorCode code)
                 case ErrorCode.WRN_ArgExpectedIn:
                 case ErrorCode.WRN_OverridingDifferentRefness:
                 case ErrorCode.WRN_HidingDifferentRefness:
+                case ErrorCode.WRN_RefReadonlyParameterDefaultValue:
                     return 1;
                 default:
                     return 0;
@@ -2374,6 +2375,7 @@ internal static bool IsBuildOnlyDiagnostic(ErrorCode code)
                 case ErrorCode.WRN_OverridingDifferentRefness:
                 case ErrorCode.WRN_HidingDifferentRefness:
                 case ErrorCode.ERR_OutAttrOnRefReadonlyParam:
+                case ErrorCode.WRN_RefReadonlyParameterDefaultValue:
                     return false;
                 default:
                     // NOTE: All error codes must be explicitly handled in this switch statement

@@ -151,7 +151,7 @@ public bool IsOptional
                 RefKind refKind;
                 return !IsParams && IsMetadataOptional &&
                        ((refKind = RefKind) == RefKind.None ||
-                        (refKind == RefKind.In) ||
+                        (refKind is RefKind.In or RefKind.RefReadOnlyParameter) ||
                         (refKind == RefKind.Ref && ContainingSymbol.ContainingType.IsComImport));
             }
         }

@@ -853,6 +853,12 @@ internal static bool ReportDefaultParameterErrors(
                     parameterSyntax.Identifier.ValueText);
             }
 
+            if (refKind == RefKind.RefReadOnlyParameter)
+            {
+                // A default value is specified for 'ref readonly' parameter '{0}', but 'ref readonly' should be used only for references. Consider declaring the parameter as 'in'.
+                diagnostics.Add(ErrorCode.WRN_RefReadonlyParameterDefaultValue, parameterSyntax.Default.Value, parameterSyntax.Identifier.ValueText);
+            }
+
             return hasErrors;
         }
 

@@ -891,6 +891,12 @@ private void DecodeDefaultParameterValueAttribute(AttributeDescription descripti
             if (!value.IsBad)
             {
                 VerifyParamDefaultValueMatchesAttributeIfAny(value, syntax, diagnostics);
+
+                if (this.RefKind == RefKind.RefReadOnlyParameter && this.IsOptional && this.CSharpSyntaxNode.Default is null)
+                {
+                    // A default value is specified for 'ref readonly' parameter '{0}', but 'ref readonly' should be used only for references. Consider declaring the parameter as 'in'.
+                    diagnostics.Add(ErrorCode.WRN_RefReadonlyParameterDefaultValue, syntax, this.Name);
+                }
             }
         }