Check generic construction diagnostics for stackalloc to span conversions

docs/compilers/CSharp/Compiler Breaking Changes - post VS2017.md

@@ -97,3 +97,4 @@ Example: `Func<int> f = default(TypedReference).GetHashCode; // new error CS0123
     This is changed in 15.6 to now produce an error that the variable is not definitely assigned.
 
 - Visual Studio 2017 version 15.7: https://github.com/dotnet/roslyn/issues/19792 C# compiler will now reject [IsReadOnly] symbols that should have an [InAttribute] modreq, but don't.
+- Visual Studio 2017 version 15.7: https://github.com/dotnet/roslyn/pull/25131 C# compiler will now check `stackalloc T [count]` expressions to see if T matches constraints of `Span<T>`.

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

@@ -3127,7 +3127,13 @@ private BoundExpression BindStackAllocArrayCreationExpression(
                 var spanType = GetWellKnownType(WellKnownType.System_Span_T, diagnostics, node);
                 if (!spanType.IsErrorType())
                 {
-                    type = spanType.Construct(elementType);
+                    type = ConstructNamedType(
+                        type: spanType,
+                        typeSyntax: node.Type,
+                        typeArgumentsSyntax: default,
+                        typeArguments: ImmutableArray.Create(elementType),
+                        basesBeingResolved: null,
+                        diagnostics: diagnostics);
                 }
             }
 

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

@@ -675,5 +675,72 @@ public static explicit operator Test(Span<int> value)
     }
 }", TestOptions.ReleaseDll).VerifyDiagnostics();
         }
+
+        [Fact]
+        [WorkItem(25038, "https://github.com/dotnet/roslyn/issues/25038")]
+        public void StackAllocToSpanWithRefStructType()
+        {
+            CreateCompilationWithMscorlibAndSpan(@"
+using System;
+ref struct S {}
+class Test
+{
+    void M()
+    {
+        Span<S> explicitError = default;
+        var implicitError = explicitError.Length > 0 ? stackalloc S[10] : stackalloc S[100];
+    }
+}").VerifyDiagnostics(
+                // (8,14): error CS0306: The type 'S' may not be used as a type argument
+                //         Span<S> explicitError = default;
+                Diagnostic(ErrorCode.ERR_BadTypeArgument, "S").WithArguments("S").WithLocation(8, 14),
+                // (9,67): error CS0306: The type 'S' may not be used as a type argument
+                //         var implicitError = explicitError.Length > 0 ? stackalloc S[10] : stackalloc S[100];
+                Diagnostic(ErrorCode.ERR_BadTypeArgument, "S[10]").WithArguments("S").WithLocation(9, 67),
+                // (9,86): error CS0306: The type 'S' may not be used as a type argument
+                //         var implicitError = explicitError.Length > 0 ? stackalloc S[10] : stackalloc S[100];
+                Diagnostic(ErrorCode.ERR_BadTypeArgument, "S[100]").WithArguments("S").WithLocation(9, 86));
+        }
+
+        [Fact]
+        [WorkItem(25086, "https://github.com/dotnet/roslyn/issues/25086")]
+        public void StaackAllocToSpanWithCustomSpanAndConstraints()
+        {
+            var code = @"
+using System;
+namespace System
+{
+    public unsafe readonly ref struct Span<T> where T : IComparable
+    {
+        public Span(void* ptr, int length)
+        {
+            Length = length;
+        }
+        public int Length { get; }
+    }
+}
+struct NonComparable { }
+class Test
+{
+    void M()
+    {
+        Span<NonComparable> explicitError = default;
+        var implicitError = explicitError.Length > 0 ? stackalloc NonComparable[10] : stackalloc NonComparable[100];
+    }
+}";
+
+            var references = new List<MetadataReference>() { MscorlibRef_v4_0_30316_17626, SystemCoreRef, CSharpRef };
+
+            CreateEmptyCompilation(code, references, TestOptions.UnsafeReleaseDll).VerifyDiagnostics(
+                // (19,14): error CS0315: The type 'NonComparable' cannot be used as type parameter 'T' in the generic type or method 'Span<T>'. There is no boxing conversion from 'NonComparable' to 'System.IComparable'.
+                //         Span<NonComparable> explicitError = default;
+                Diagnostic(ErrorCode.ERR_GenericConstraintNotSatisfiedValType, "NonComparable").WithArguments("System.Span<T>", "System.IComparable", "T", "NonComparable").WithLocation(19, 14),
+                // (20,67): error CS0315: The type 'NonComparable' cannot be used as type parameter 'T' in the generic type or method 'Span<T>'. There is no boxing conversion from 'NonComparable' to 'System.IComparable'.
+                //         var implicitError = explicitError.Length > 0 ? stackalloc NonComparable[10] : stackalloc NonComparable[100];
+                Diagnostic(ErrorCode.ERR_GenericConstraintNotSatisfiedValType, "NonComparable[10]").WithArguments("System.Span<T>", "System.IComparable", "T", "NonComparable").WithLocation(20, 67),
+                // (20,98): error CS0315: The type 'NonComparable' cannot be used as type parameter 'T' in the generic type or method 'Span<T>'. There is no boxing conversion from 'NonComparable' to 'System.IComparable'.
+                //         var implicitError = explicitError.Length > 0 ? stackalloc NonComparable[10] : stackalloc NonComparable[100];
+                Diagnostic(ErrorCode.ERR_GenericConstraintNotSatisfiedValType, "NonComparable[100]").WithArguments("System.Span<T>", "System.IComparable", "T", "NonComparable").WithLocation(20, 98));
+        }
     }
 }