Emit CompilerFeatureRequiredAttribute for instance operators

This prevents old compilers and other compilers (VB, etc.) from consuming the new APIs in an unintended way.

Author: AlekseyTs

src/Compilers/CSharp/Portable/Symbols/Metadata/PE/PEMethodSymbol.cs

@@ -1010,18 +1010,19 @@ public override ImmutableArray<CSharpAttributeData> GetAttributes()
                      : IsValidReadOnlyTarget;
 
                 bool checkForRequiredMembers = this.ShouldCheckRequiredMembers() && this.ContainingType.HasAnyRequiredMembers;
+                bool isInstanceIncrementDecrementOrCompoundAssignmentOperator = SourceMethodSymbol.IsInstanceIncrementDecrementOrCompoundAssignmentOperator(this);
 
                 bool isExtensionMethod = false;
                 bool isReadOnly = false;
-                if (checkForExtension || checkForIsReadOnly || checkForRequiredMembers)
+                if (checkForExtension || checkForIsReadOnly || checkForRequiredMembers || isInstanceIncrementDecrementOrCompoundAssignmentOperator)
                 {
                     attributeData = containingPEModuleSymbol.GetCustomAttributesForToken(_handle,
                         filteredOutAttribute1: out CustomAttributeHandle extensionAttribute,
                         filterOut1: AttributeDescription.CaseSensitiveExtensionAttribute,
                         filteredOutAttribute2: out CustomAttributeHandle isReadOnlyAttribute,
                         filterOut2: AttributeDescription.IsReadOnlyAttribute,
                         filteredOutAttribute3: out _,
-                        filterOut3: (checkForRequiredMembers && DeriveCompilerFeatureRequiredDiagnostic() is null) ? AttributeDescription.CompilerFeatureRequiredAttribute : default,
+                        filterOut3: ((checkForRequiredMembers || isInstanceIncrementDecrementOrCompoundAssignmentOperator) && DeriveCompilerFeatureRequiredDiagnostic() is null) ? AttributeDescription.CompilerFeatureRequiredAttribute : default,
                         filteredOutAttribute4: out _,
                         filterOut4: (checkForRequiredMembers && ObsoleteAttributeData is null) ? AttributeDescription.ObsoleteAttribute : default,
                         filteredOutAttribute5: out _,
@@ -1520,7 +1521,14 @@ private DiagnosticInfo DeriveCompilerFeatureRequiredDiagnostic()
         {
             var containingModule = _containingType.ContainingPEModule;
             var decoder = new MetadataDecoder(containingModule, this);
-            var diag = PEUtilities.DeriveCompilerFeatureRequiredAttributeDiagnostic(this, containingModule, Handle, allowedFeatures: MethodKind == MethodKind.Constructor ? CompilerFeatureRequiredFeatures.RequiredMembers : CompilerFeatureRequiredFeatures.None, decoder);
+            var diag = PEUtilities.DeriveCompilerFeatureRequiredAttributeDiagnostic(
+                this, containingModule, Handle,
+                allowedFeatures: MethodKind == MethodKind.Constructor ?
+                    CompilerFeatureRequiredFeatures.RequiredMembers :
+                    (SourceMethodSymbol.IsInstanceIncrementDecrementOrCompoundAssignmentOperator(this) ?
+                        CompilerFeatureRequiredFeatures.UserDefinedCompoundAssignmentOperators :
+                        CompilerFeatureRequiredFeatures.None),
+                decoder);
 
             if (diag != null)
             {

src/Compilers/CSharp/Portable/Symbols/Source/SourceMethodSymbol.cs

@@ -224,6 +224,26 @@ SynthesizedEventAccessorSymbol or
             {
                 AddSynthesizedAttribute(ref attributes, compilation.TrySynthesizeAttribute(WellKnownMember.System_Diagnostics_DebuggerHiddenAttribute__ctor));
             }
+
+            if (IsInstanceIncrementDecrementOrCompoundAssignmentOperator(target))
+            {
+                AddSynthesizedAttribute(ref attributes, compilation.TrySynthesizeAttribute(WellKnownMember.System_Runtime_CompilerServices_CompilerFeatureRequiredAttribute__ctor,
+                    ImmutableArray.Create(new TypedConstant(compilation.GetSpecialType(SpecialType.System_String), TypedConstantKind.Primitive, nameof(CompilerFeatureRequiredFeatures.UserDefinedCompoundAssignmentOperators)))
+                    ));
+            }
+        }
+
+        internal static bool IsInstanceIncrementDecrementOrCompoundAssignmentOperator(MethodSymbol target)
+        {
+            if (target.MethodKind == MethodKind.UserDefinedOperator && !target.IsStatic)
+            {
+                SyntaxKind syntaxKind = SyntaxFacts.GetOperatorKind(target.Name);
+
+                return syntaxKind is (SyntaxKind.PlusPlusToken or SyntaxKind.MinusMinusToken) ||
+                       SyntaxFacts.IsOverloadableCompoundAssignmentOperator(syntaxKind);
+            }
+
+            return false;
         }
     }
 }

src/Compilers/CSharp/Portable/Symbols/Source/SourceUserDefinedOperatorSymbolBase.cs

@@ -181,6 +181,11 @@ protected SourceUserDefinedOperatorSymbolBase(
             // SPEC: It is an error for the same modifier to appear multiple times in an
             // SPEC: operator declaration.
             ModifierUtils.CheckAccessibility(this.DeclarationModifiers, this, isExplicitInterfaceImplementation: false, diagnostics, location);
+
+            if (!IsStatic && (isIncrementDecrement || isCompoundAssignment))
+            {
+                _ = Binder.GetWellKnownTypeMember(DeclaringCompilation, WellKnownMember.System_Runtime_CompilerServices_CompilerFeatureRequiredAttribute__ctor, diagnostics, location);
+            }
         }
 
         private static (bool isIncrementDecrement, bool isCompoundAssignment) IsAssignmentOperatorDeclaration(CSharpSyntaxNode syntax)