Propagate params to lambdas and local functions

docs/compilers/CSharp/Compiler Breaking Changes - DotNet 10.md

@@ -407,3 +407,30 @@ partial interface I
 
 class C : I;
 ```
+
+## Missing `ParamCollectionAttribute` is reported in more cases
+
+***Introduced in Visual Studio 2026 version 18.0***
+
+If you are compiling a `.netmodule` (note that this doesn't apply to normal DLL/EXE compilations),
+and have a lambda or a local function with a `params` collection parameter,
+and the `ParamCollectionAttribute` is not found, a compilation error is now reported
+(because the attribute now must be [emitted](https://github.com/dotnet/roslyn/issues/79752) on the synthesized method
+but the attribute type itself is not synthesized by the compiler into a `.netmodule`).
+You can work around that by defining the attribute yourself.
+
+```cs
+using System;
+using System.Collections.Generic;
+class C
+{
+    void M()
+    {
+        Func<IList<int>, int> lam = (params IList<int> xs) => xs.Count; // error if ParamCollectionAttribute does not exist
+        lam([1, 2, 3]);
+
+        int func(params IList<int> xs) => xs.Count; // error if ParamCollectionAttribute does not exist
+        func(4, 5, 6);
+    }
+}
+```

src/Compilers/CSharp/Portable/BoundTree/UnboundLambda.cs

@@ -843,7 +843,7 @@ private BoundLambda ReallyBind(NamedTypeSymbol delegateType, bool inExpressionTr
 
             var lambdaParameters = lambdaSymbol.Parameters;
             ParameterHelpers.EnsureRefKindAttributesExist(compilation, lambdaParameters, diagnostics, modifyCompilation: false);
-            // Not emitting ParamCollectionAttribute/ParamArrayAttribute for lambdas
+            ParameterHelpers.EnsureParamCollectionAttributeExists(compilation, lambdaParameters, diagnostics, modifyCompilation: false);
 
             if (returnType.HasType)
             {

src/Compilers/CSharp/Portable/Lowering/ClosureConversion/SynthesizedClosureMethod.cs

@@ -122,7 +122,8 @@ private void EnsureAttributesExist(TypeCompilationState compilationState)
             }
 
             ParameterHelpers.EnsureRefKindAttributesExist(moduleBuilder, Parameters);
-            // Not emitting ParamCollectionAttribute/ParamArrayAttribute for these methods because it is not a SynthesizedDelegateInvokeMethod
+
+            ParameterHelpers.EnsureParamCollectionAttributeExists(moduleBuilder, Parameters);
 
             if (moduleBuilder.Compilation.ShouldEmitNativeIntegerAttributes())
             {

src/Compilers/CSharp/Portable/Lowering/SynthesizedMethodBaseSymbol.cs

@@ -129,7 +129,8 @@ private ImmutableArray<ParameterSymbol> MakeParameters()
                     p.ExplicitDefaultConstantValue,
                     // the synthesized parameter doesn't need to have the same ref custom modifiers as the base
                     refCustomModifiers: default,
-                    baseParameterForAttributes: inheritAttributes ? p : null));
+                    baseParameterForAttributes: inheritAttributes ? p : null,
+                    isParams: this is SynthesizedClosureMethod && p.IsParams));
             }
 
             var extraSynthed = ExtraSynthesizedRefParameters;

src/Compilers/CSharp/Portable/Symbols/Compilation_WellKnownMembers.cs

@@ -537,9 +537,9 @@ internal void EnsureRequiresLocationAttributeExists(BindingDiagnosticBag? diagno
             EnsureEmbeddableAttributeExists(EmbeddableAttributes.RequiresLocationAttribute, diagnostics, location, modifyCompilation);
         }
 
-        internal void EnsureParamCollectionAttributeExistsAndModifyCompilation(BindingDiagnosticBag? diagnostics, Location location)
+        internal void EnsureParamCollectionAttributeExists(BindingDiagnosticBag? diagnostics, Location location, bool modifyCompilation)
         {
-            EnsureEmbeddableAttributeExists(EmbeddableAttributes.ParamCollectionAttribute, diagnostics, location, modifyCompilation: true);
+            EnsureEmbeddableAttributeExists(EmbeddableAttributes.ParamCollectionAttribute, diagnostics, location, modifyCompilation: modifyCompilation);
         }
 
         internal void EnsureIsByRefLikeAttributeExists(BindingDiagnosticBag? diagnostics, Location location, bool modifyCompilation)

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

@@ -126,7 +126,7 @@ internal void GetDeclarationDiagnostics(BindingDiagnosticBag addTo)
 
             var compilation = DeclaringCompilation;
             ParameterHelpers.EnsureRefKindAttributesExist(compilation, Parameters, addTo, modifyCompilation: false);
-            // Not emitting ParamCollectionAttribute/ParamArrayAttribute for local functions
+            ParameterHelpers.EnsureParamCollectionAttributeExists(compilation, Parameters, addTo, modifyCompilation: false);
             ParameterHelpers.EnsureNativeIntegerAttributeExists(compilation, Parameters, addTo, modifyCompilation: false);
             ParameterHelpers.EnsureScopedRefAttributeExists(compilation, Parameters, addTo, modifyCompilation: false);
             ParameterHelpers.EnsureNullableAttributeExists(compilation, this, Parameters, addTo, modifyCompilation: false);

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

@@ -370,11 +370,19 @@ private static void EnsureRefKindAttributesExist(CSharpCompilation compilation,
             }
         }
 
-        internal static void EnsureParamCollectionAttributeExistsAndModifyCompilation(CSharpCompilation compilation, ImmutableArray<ParameterSymbol> parameters, BindingDiagnosticBag diagnostics)
+        internal static void EnsureParamCollectionAttributeExists(PEModuleBuilder moduleBuilder, ImmutableArray<ParameterSymbol> parameters)
         {
             if (parameters.LastOrDefault(static (p) => p.IsParamsCollection) is { } parameter)
             {
-                compilation.EnsureParamCollectionAttributeExistsAndModifyCompilation(diagnostics, GetParameterLocation(parameter));
+                moduleBuilder.EnsureParamCollectionAttributeExists(null, null);
+            }
+        }
+
+        internal static void EnsureParamCollectionAttributeExists(CSharpCompilation compilation, ImmutableArray<ParameterSymbol> parameters, BindingDiagnosticBag diagnostics, bool modifyCompilation)
+        {
+            if (parameters.LastOrDefault(static (p) => p.IsParamsCollection) is { } parameter)
+            {
+                compilation.EnsureParamCollectionAttributeExists(diagnostics, GetParameterLocation(parameter), modifyCompilation);
             }
         }
 

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

@@ -89,7 +89,7 @@ internal sealed override void AfterAddingTypeMembersChecks(ConversionsBase conve
 
             var compilation = DeclaringCompilation;
             ParameterHelpers.EnsureRefKindAttributesExist(compilation, Parameters, diagnostics, modifyCompilation: true);
-            ParameterHelpers.EnsureParamCollectionAttributeExistsAndModifyCompilation(compilation, Parameters, diagnostics);
+            ParameterHelpers.EnsureParamCollectionAttributeExists(compilation, Parameters, diagnostics, modifyCompilation: true);
             ParameterHelpers.EnsureNativeIntegerAttributeExists(compilation, Parameters, diagnostics, modifyCompilation: true);
             ParameterHelpers.EnsureScopedRefAttributeExists(compilation, Parameters, diagnostics, modifyCompilation: true);
             ParameterHelpers.EnsureNullableAttributeExists(compilation, this, Parameters, diagnostics, modifyCompilation: true);

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

@@ -323,7 +323,7 @@ internal override void AfterAddingTypeMembersChecks(ConversionsBase conversions,
                 }
 
                 ParameterHelpers.EnsureRefKindAttributesExist(compilation, Parameters, diagnostics, modifyCompilation: true);
-                ParameterHelpers.EnsureParamCollectionAttributeExistsAndModifyCompilation(compilation, Parameters, diagnostics);
+                ParameterHelpers.EnsureParamCollectionAttributeExists(compilation, Parameters, diagnostics, modifyCompilation: true);
 
                 if (compilation.ShouldEmitNativeIntegerAttributes(ReturnType))
                 {

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

@@ -245,7 +245,7 @@ internal override void AfterAddingTypeMembersChecks(ConversionsBase conversions,
             }
 
             ParameterHelpers.EnsureRefKindAttributesExist(compilation, Parameters, diagnostics, modifyCompilation: true);
-            ParameterHelpers.EnsureParamCollectionAttributeExistsAndModifyCompilation(compilation, Parameters, diagnostics);
+            ParameterHelpers.EnsureParamCollectionAttributeExists(compilation, Parameters, diagnostics, modifyCompilation: true);
 
             if (compilation.ShouldEmitNativeIntegerAttributes(ReturnType))
             {

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

@@ -1044,7 +1044,7 @@ internal override void AfterAddingTypeMembersChecks(ConversionsBase conversions,
             }
 
             ParameterHelpers.EnsureRefKindAttributesExist(compilation, Parameters, diagnostics, modifyCompilation: true);
-            ParameterHelpers.EnsureParamCollectionAttributeExistsAndModifyCompilation(compilation, Parameters, diagnostics);
+            ParameterHelpers.EnsureParamCollectionAttributeExists(compilation, Parameters, diagnostics, modifyCompilation: true);
 
             if (compilation.ShouldEmitNativeIntegerAttributes(Type))
             {

src/Compilers/CSharp/Portable/Symbols/Synthesized/SynthesizedParameterSymbol.cs

@@ -194,11 +194,11 @@ internal override void AddSynthesizedAttributes(PEModuleBuilder moduleBuilder, r
                 AddSynthesizedAttribute(ref attributes, compilation.TrySynthesizeAttribute(WellKnownMember.System_Diagnostics_CodeAnalysis_UnscopedRefAttribute__ctor));
             }
 
-            if (this.IsParamsArray && this.ContainingSymbol is SynthesizedDelegateInvokeMethod)
+            if (this.IsParamsArray)
             {
                 AddSynthesizedAttribute(ref attributes, compilation.TrySynthesizeAttribute(WellKnownMember.System_ParamArrayAttribute__ctor));
             }
-            else if (this.IsParamsCollection && this.ContainingSymbol is SynthesizedDelegateInvokeMethod)
+            else if (this.IsParamsCollection)
             {
                 AddSynthesizedAttribute(ref attributes, moduleBuilder.SynthesizeParamCollectionAttribute(this));
             }
@@ -363,6 +363,8 @@ public SynthesizedComplexParameterSymbol(
             Debug.Assert(isParams || !refCustomModifiers.IsEmpty || baseParameterForAttributes is object || defaultValue is not null || hasUnscopedRefAttribute);
             Debug.Assert(baseParameterForAttributes is null || baseParameterForAttributes.ExplicitDefaultConstantValue == defaultValue);
             Debug.Assert(baseParameterForAttributes is null || baseParameterForAttributes.RefKind == refKind);
+            Debug.Assert(!isParams || container is SynthesizedDelegateInvokeMethod or SynthesizedClosureMethod,
+                "If this fails, make sure we don't synthesize ParamsArrayAttribute for symbols we don't intend to.");
 
             _refCustomModifiers = refCustomModifiers;
             _baseParameterForAttributes = baseParameterForAttributes;

src/Compilers/CSharp/Test/Emit3/Semantics/ParamsCollectionTests.cs

@@ -20,15 +20,7 @@ namespace Microsoft.CodeAnalysis.CSharp.UnitTests.Semantics
 {
     public class ParamsCollectionTests : CompilingTestBase
     {
-        private const string ParamCollectionAttributeSource = @"
-namespace System.Runtime.CompilerServices
-{
-    public sealed class ParamCollectionAttribute : Attribute
-    {
-        public ParamCollectionAttribute() { }
-    }
-}
-";
+        private static string ParamCollectionAttributeSource => TestSources.ParamsCollectionAttribute;
 
         private static void VerifyParamsAndAttribute(ParameterSymbol parameter, bool isParamArray = false, bool isParamCollection = false)
         {
@@ -2741,10 +2733,8 @@ static void Main()
                 symbolValidator: (m) =>
                 {
                     MethodSymbol l1 = m.GlobalNamespace.GetMember<MethodSymbol>("Program.<>c.<Main>b__0_0");
-                    AssertEx.Equal("void Program.<>c.<Main>b__0_0(System.Collections.Generic.IEnumerable<System.Int64> x)", l1.ToTestDisplayString());
-                    VerifyParamsAndAttribute(l1.Parameters.Last());
-
-                    Assert.Empty(((NamespaceSymbol)m.GlobalNamespace.GetMember("System.Runtime.CompilerServices")).GetMembers("ParamCollectionAttribute"));
+                    AssertEx.Equal("void Program.<>c.<Main>b__0_0(params System.Collections.Generic.IEnumerable<System.Int64> x)", l1.ToTestDisplayString());
+                    VerifyParamsAndAttribute(l1.Parameters.Last(), isParamCollection: true);
                 }).VerifyDiagnostics(
                     // (7,72): warning CS9100: Parameter 1 has params modifier in lambda but not in target delegate type.
                     //         System.Action<IEnumerable<long>> l = (params IEnumerable<long> x) => {};
@@ -2797,8 +2787,8 @@ static void Main()
                 symbolValidator: (m) =>
                 {
                     MethodSymbol l1 = m.GlobalNamespace.GetMember<MethodSymbol>("Program.<>c.<Main>b__0_0");
-                    AssertEx.Equal("void Program.<>c.<Main>b__0_0(System.Int64[] x)", l1.ToTestDisplayString());
-                    VerifyParamsAndAttribute(l1.Parameters.Last());
+                    AssertEx.Equal("void Program.<>c.<Main>b__0_0(params System.Int64[] x)", l1.ToTestDisplayString());
+                    VerifyParamsAndAttribute(l1.Parameters.Last(), isParamArray: true);
                 }).VerifyDiagnostics(
                     // (5,50): warning CS9100: Parameter 1 has params modifier in lambda but not in target delegate type.
                     //         System.Action<long[]> l = (params long[] x) => {};
@@ -2943,10 +2933,8 @@ static void Main()
                 symbolValidator: (m) =>
                 {
                     MethodSymbol l1 = m.GlobalNamespace.GetMember<MethodSymbol>("Program.<Main>g__local|0_0");
-                    AssertEx.Equal("void Program.<Main>g__local|0_0(System.Collections.Generic.IEnumerable<System.Int64> x)", l1.ToTestDisplayString());
-                    VerifyParamsAndAttribute(l1.Parameters.Last());
-
-                    Assert.Empty(((NamespaceSymbol)m.GlobalNamespace.GetMember("System.Runtime.CompilerServices")).GetMembers("ParamCollectionAttribute"));
+                    AssertEx.Equal("void Program.<Main>g__local|0_0(params System.Collections.Generic.IEnumerable<System.Int64> x)", l1.ToTestDisplayString());
+                    VerifyParamsAndAttribute(l1.Parameters.Last(), isParamCollection: true);
                 }).VerifyDiagnostics();
 
             var tree = comp.SyntaxTrees.Single();
@@ -2992,8 +2980,8 @@ static void Main()
                 symbolValidator: (m) =>
                 {
                     MethodSymbol l1 = m.GlobalNamespace.GetMember<MethodSymbol>("Program.<Main>g__local|0_0");
-                    AssertEx.Equal("void Program.<Main>g__local|0_0(System.Int64[] x)", l1.ToTestDisplayString());
-                    VerifyParamsAndAttribute(l1.Parameters.Last());
+                    AssertEx.Equal("void Program.<Main>g__local|0_0(params System.Int64[] x)", l1.ToTestDisplayString());
+                    VerifyParamsAndAttribute(l1.Parameters.Last(), isParamArray: true);
                 }).VerifyDiagnostics();
         }
 
@@ -3078,7 +3066,7 @@ .locals init (<>y__InlineArray2<CustomHandler> V_0,
   IL_006c:  ldloca.s   V_0
   IL_006e:  ldc.i4.2
   IL_006f:  call       ""System.ReadOnlySpan<CustomHandler> <PrivateImplementationDetails>.InlineArrayAsReadOnlySpan<<>y__InlineArray2<CustomHandler>, CustomHandler>(in <>y__InlineArray2<CustomHandler>, int)""
-  IL_0074:  call       ""void Program.<<Main>$>g__M|0_0(scoped System.ReadOnlySpan<CustomHandler>)""
+  IL_0074:  call       ""void Program.<<Main>$>g__M|0_0(params System.ReadOnlySpan<CustomHandler>)""
   IL_0079:  ret
 }
 ");
@@ -4533,15 +4521,13 @@ void verify(CSharpCompilation comp, bool attributeIsEmbedded)
                         AssertEx.Equal("void <>f__AnonymousDelegate1<T1>.Invoke(params T1[] arg)", delegateInvokeMethod2.ToTestDisplayString());
                         VerifyParamsAndAttribute(delegateInvokeMethod2.Parameters.Last(), isParamArray: true);
 
-                        // Note, no attributes on lambdas
-
                         MethodSymbol l1 = m.GlobalNamespace.GetMember<MethodSymbol>("Program.<>c.<Main>b__0_0");
-                        AssertEx.Equal("void Program.<>c.<Main>b__0_0(scoped System.ReadOnlySpan<System.Int64> a)", l1.ToTestDisplayString());
-                        VerifyParamsAndAttribute(l1.Parameters.Last());
+                        AssertEx.Equal("void Program.<>c.<Main>b__0_0(params System.ReadOnlySpan<System.Int64> a)", l1.ToTestDisplayString());
+                        VerifyParamsAndAttribute(l1.Parameters.Last(), isParamCollection: true);
 
                         MethodSymbol l2 = m.GlobalNamespace.GetMember<MethodSymbol>("Program.<>c.<Main>b__0_1");
-                        AssertEx.Equal("void Program.<>c.<Main>b__0_1(System.Int64[] a)", l2.ToTestDisplayString());
-                        VerifyParamsAndAttribute(l2.Parameters.Last());
+                        AssertEx.Equal("void Program.<>c.<Main>b__0_1(params System.Int64[] a)", l2.ToTestDisplayString());
+                        VerifyParamsAndAttribute(l2.Parameters.Last(), isParamArray: true);
 
                         if (attributeIsEmbedded)
                         {
@@ -4686,15 +4672,13 @@ void verify(CSharpCompilation comp, bool attributeIsEmbedded)
                         AssertEx.Equal("void <>f__AnonymousDelegate1<T1>.Invoke(params T1[] arg)", delegateInvokeMethod2.ToTestDisplayString());
                         VerifyParamsAndAttribute(delegateInvokeMethod2.Parameters.Last(), isParamArray: true);
 
-                        // Note, no attributes on local functions
-
                         MethodSymbol l1 = m.GlobalNamespace.GetMember<MethodSymbol>("Program.<Main>g__Test1|0_0");
-                        AssertEx.Equal("void Program.<Main>g__Test1|0_0(scoped System.ReadOnlySpan<System.Int64> a)", l1.ToTestDisplayString());
-                        VerifyParamsAndAttribute(l1.Parameters.Last());
+                        AssertEx.Equal("void Program.<Main>g__Test1|0_0(params System.ReadOnlySpan<System.Int64> a)", l1.ToTestDisplayString());
+                        VerifyParamsAndAttribute(l1.Parameters.Last(), isParamCollection: true);
 
                         MethodSymbol l2 = m.GlobalNamespace.GetMember<MethodSymbol>("Program.<Main>g__Test2|0_1");
-                        AssertEx.Equal("void Program.<Main>g__Test2|0_1(System.Int64[] a)", l2.ToTestDisplayString());
-                        VerifyParamsAndAttribute(l2.Parameters.Last());
+                        AssertEx.Equal("void Program.<Main>g__Test2|0_1(params System.Int64[] a)", l2.ToTestDisplayString());
+                        VerifyParamsAndAttribute(l2.Parameters.Last(), isParamArray: true);
 
                         if (attributeIsEmbedded)
                         {
@@ -12970,9 +12954,9 @@ void Test1(params IEnumerable<long> a) { }
                 src,
                 "<>f__AnonymousDelegate0.Invoke",
                 "void <>f__AnonymousDelegate0.Invoke(params System.Collections.Generic.IEnumerable<System.Int64> arg)",
-                // (7,18): error CS0518: Predefined type 'System.Runtime.CompilerServices.ParamCollectionAttribute' is not defined or imported
-                //         var x1 = Test1;
-                Diagnostic(ErrorCode.ERR_PredefinedTypeNotFound, "Test1").WithArguments("System.Runtime.CompilerServices.ParamCollectionAttribute").WithLocation(7, 18)
+                // (11,20): error CS0518: Predefined type 'System.Runtime.CompilerServices.ParamCollectionAttribute' is not defined or imported
+                //         void Test1(params IEnumerable<long> a) { }
+                Diagnostic(ErrorCode.ERR_PredefinedTypeNotFound, "params IEnumerable<long> a").WithArguments("System.Runtime.CompilerServices.ParamCollectionAttribute").WithLocation(11, 20)
                 );
         }
 

src/Compilers/CSharp/Test/Semantic/Semantics/DelegateTypeTests.cs

@@ -19463,6 +19463,10 @@ .method assembly hidebysig
                 			int32[] ys
                 		) cil managed 
                 	{
+                		.param [2]
+                			.custom instance void [{{s_libPrefix}}]System.ParamArrayAttribute::.ctor() = (
+                				01 00 00 00
+                			)
                 		// Method begins at RVA 0x20b0
                 		// Code size 1 (0x1)
                 		.maxstack 8
@@ -19520,6 +19524,10 @@ int32[] ys
                 		.custom instance void [{{s_libPrefix}}]System.Runtime.CompilerServices.CompilerGeneratedAttribute::.ctor() = (
                 			01 00 00 00
                 		)
+                		.param [2]
+                			.custom instance void [{{s_libPrefix}}]System.ParamArrayAttribute::.ctor() = (
+                				01 00 00 00
+                			)
                 		// Method begins at RVA 0x2067
                 		// Code size 1 (0x1)
                 		.maxstack 8