Forbid types with required members from substituting for where T : new()

Specification: https://github.com/dotnet/csharplang/blob/main/proposals/required-members.md
Test plan: dotnet#57046

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

@@ -1378,16 +1378,17 @@ private static bool HasPublicParameterlessConstructor(NamedTypeSymbol type, bool
         {
             Debug.Assert(type.TypeKind is TypeKind.Class or TypeKind.Struct);
 
-            // PROTOTYPE(req): Adjust for required members
+            bool hasAnyRequiredMembers = type.HasAnyRequiredMembers;
 
             foreach (var constructor in type.InstanceConstructors)
             {
                 if (constructor.ParameterCount == 0)
                 {
-                    return constructor.DeclaredAccessibility == Accessibility.Public;
+                    return constructor.DeclaredAccessibility == Accessibility.Public
+                        && (!hasAnyRequiredMembers || constructor.HasSetsRequiredMembers);
                 }
             }
-            return synthesizedIfMissing;
+            return synthesizedIfMissing && !hasAnyRequiredMembers;
         }
 
         /// <summary>

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

@@ -523,13 +523,20 @@ internal bool HasRequiredMembersError
             }
         }
 
+        /// <summary>
+        /// Returns true if there are any required members. Prefer calling this over checking <see cref="AllRequiredMembers"/> for empty, as
+        /// this will avoid calculating base type requirements if not necessary.
+        /// </summary>
+        internal bool HasAnyRequiredMembers => HasDeclaredRequiredMembers || !AllRequiredMembers.IsEmpty;
+
         /// <summary>
         /// The full list of all required members for this type, including from base classes. If <see cref="HasRequiredMembersError"/> is true,
         /// this returns empty.
         /// </summary>
         /// <remarks>
         /// Do not call this API if all you need are the required members declared on this type. Use <see cref="GetMembers()"/> instead, filtering for
-        /// required members, instead of calling this API.
+        /// required members, instead of calling this API. If you only need to determine whether this type or any base types have required members, call
+        /// <see cref="HasAnyRequiredMembers"/>, which will avoid calling this API if not required.
         /// </remarks>
         internal ImmutableSegmentedDictionary<string, Symbol> AllRequiredMembers
         {

src/Compilers/CSharp/Portable/Symbols/Synthesized/Records/SynthesizedRecordCopyCtor.cs

@@ -135,6 +135,6 @@ internal static bool HasCopyConstructorSignature(MethodSymbol member)
 
         protected sealed override bool HasSetsRequiredMembersImpl
             // If the record type has a required members error, then it does have required members of some kind, we emit the SetsRequiredMembers attribute.
-            => !ContainingType.AllRequiredMembers.IsEmpty || ContainingType.HasRequiredMembersError;
+            => ContainingType.HasAnyRequiredMembers || ContainingType.HasRequiredMembersError;
     }
 }

src/Compilers/CSharp/Test/Symbol/Symbols/RequiredMembersTests.cs

@@ -4048,4 +4048,124 @@ void validate(ModuleSymbol module)
             }
         }
     }
+
+    [Theory]
+    [InlineData("struct")]
+    [InlineData("class")]
+    public void ForbidRequiredAsNew_NoInheritance(string typeKind)
+    {
+        var code = $$"""
+            M<C>();
+
+            void M<T>() where T : new()
+            {
+            }
+
+            {{typeKind}} C
+            {
+                public required int Prop1 { get; set; }
+            }
+            """;
+
+        var comp = CreateCompilationWithRequiredMembers(code);
+        comp.VerifyDiagnostics(
+            // (1,1): error CS0310: 'C' must be a non-abstract type with a public parameterless constructor in order to use it as parameter 'T' in the generic type or method 'M<T>()'
+            // M<C>();
+            Diagnostic(ErrorCode.ERR_NewConstraintNotSatisfied, "M<C>").WithArguments("M<T>()", "T", "C").WithLocation(1, 1)
+        );
+    }
+
+    [Theory]
+    [CombinatorialData]
+    public void ForbidRequiredAsNew_Inheritance(bool useMetadataReference)
+    {
+        var @base = """
+            public class Base
+            {
+                public required int Prop1 { get; set; }
+            }
+            """;
+
+        var code = """
+            M<Derived>();
+
+            void M<T>() where T : new()
+            {
+            }
+
+            class Derived : Base
+            {
+            }
+            """;
+
+        var comp = CreateCompilationWithRequiredMembers(new[] { @base, code });
+        comp.VerifyDiagnostics(
+            // (1,1): error CS0310: 'Derived' must be a non-abstract type with a public parameterless constructor in order to use it as parameter 'T' in the generic type or method 'M<T>()'
+            // M<Derived>();
+            Diagnostic(ErrorCode.ERR_NewConstraintNotSatisfied, "M<Derived>").WithArguments("M<T>()", "T", "Derived").WithLocation(1, 1)
+        );
+
+        var baseComp = CreateCompilationWithRequiredMembers(@base);
+        CompileAndVerify(baseComp).VerifyDiagnostics();
+
+        comp = CreateCompilation(code, references: new[] { useMetadataReference ? baseComp.ToMetadataReference() : baseComp.EmitToImageReference() });
+        comp.VerifyDiagnostics(
+            // (1,1): error CS0310: 'Derived' must be a non-abstract type with a public parameterless constructor in order to use it as parameter 'T' in the generic type or method 'M<T>()'
+            // M<Derived>();
+            Diagnostic(ErrorCode.ERR_NewConstraintNotSatisfied, "M<Derived>").WithArguments("M<T>()", "T", "Derived").WithLocation(1, 1)
+        );
+    }
+
+    [Theory]
+    [InlineData("class")]
+    [InlineData("struct")]
+    public void AllowRequiredAsNew_SetsRequiredMembersOnConstructor(string typeKind)
+    {
+        var code = $$"""
+            using System.Diagnostics.CodeAnalysis;
+            M<C>();
+
+            void M<T>() where T : new()
+            {
+            }
+
+            {{typeKind}} C
+            {
+                public required int Prop1 { get; set; }
+
+                [SetsRequiredMembers]
+                public C()
+                {
+                }
+            }
+            """;
+
+        var comp = CreateCompilationWithRequiredMembers(code);
+        CompileAndVerify(comp).VerifyDiagnostics();
+    }
+
+    [Fact]
+    public void AllowRequiredAsNew_IndirectionViaStruct()
+    {
+        var code = """
+            M1<C>();
+
+            void M1<T>() where T : struct
+            {
+                M2<T>();
+            }
+
+            void M2<T>() where T : new()
+            {
+            }
+
+            struct C
+            {
+                public required int Prop1 { get; set; }
+            }
+            """;
+
+        var comp = CreateCompilationWithRequiredMembers(code);
+        CompileAndVerify(comp).VerifyDiagnostics();
+    }
 }