Pin params, ref and out interaction with span conversions

The remaining parameter-modifier dimension of the C# 14 rules: an expanded
params call prefers the params ReadOnlySpan overload over params array (the
C# 13 better-params-collection rule) while the normal form keeps the exact
array overload; and a span conversion never binds a ref or out parameter,
so a keyword-less argument picks the by-value overload and the ref/out
keyword must survive decompilation to keep binding the by-ref one. All
expectations come from compiling probes (the negative directions are
CS1503) and everything was green as written - these are lock-downs, not
fixes.

Part of #829.

Assisted-by: Claude:claude-fable-5:Claude Code

ICSharpCode.Decompiler.Tests/Semantics/OverloadResolutionTests.cs

@@ -402,17 +402,20 @@ namespace ICSharpCode.Decompiler.Tests.Semantics
 			Assert.That(r.IsAmbiguous);
 		}
 
-		static IMethod MakeInMethodIn(ICompilation c, Type parameterType)
+		static IMethod MakeByRefMethodIn(ICompilation c, Type parameterType, ReferenceKind kind)
 		{
 			var m = new FakeMethod(c, SymbolKind.Method);
 			m.Name = "Method";
 			m.Parameters = new List<IParameter> {
 				new DefaultParameter(new ByReferenceType(c.FindType(parameterType)), string.Empty,
-					owner: m, referenceKind: ReferenceKind.In)
+					owner: m, referenceKind: kind)
 			};
 			return m;
 		}
 
+		static IMethod MakeInMethodIn(ICompilation c, Type parameterType)
+			=> MakeByRefMethodIn(c, parameterType, ReferenceKind.In);
+
 		[Test]
 		public void InReadOnlySpanParameter_BindsAnArrayWithoutInButNotWithIn()
 		{
@@ -472,6 +475,29 @@ namespace ICSharpCode.Decompiler.Tests.Semantics
 			}
 		}
 
+		[Test]
+		public void RefAndOutParametersNeverBindThroughASpanConversion()
+		{
+			// Roslyn: CS1503 for both 'M(ref arr)' against 'ref ReadOnlySpan<int>' and
+			// 'M(out arr)' against 'out ReadOnlySpan<int>' - ref and out demand the
+			// parameter's own type; the span conversion does not apply. A value argument
+			// without the keyword is a passing-mode mismatch regardless of conversions.
+			var c = spanCompilation.Value;
+			var arrayArg = new ResolveResult(new ArrayType(c, c.FindType(KnownTypeCode.Int32)));
+			foreach (var kind in new[] { ReferenceKind.Ref, ReferenceKind.Out })
+			{
+				var byRefArgument = new OverloadResolution(c, new[] {
+					new ByReferenceResolveResult(arrayArg, kind)
+				});
+				Assert.That(byRefArgument.AddCandidate(MakeByRefMethodIn(c, typeof(ReadOnlySpan<int>), kind)),
+					Is.Not.EqualTo(OverloadResolutionErrors.None), kind.ToString());
+
+				var valueArgument = new OverloadResolution(c, new[] { arrayArg });
+				Assert.That(valueArgument.AddCandidate(MakeByRefMethodIn(c, typeof(ReadOnlySpan<int>), kind)),
+					Is.Not.EqualTo(OverloadResolutionErrors.None), kind.ToString());
+			}
+		}
+
 		[Test]
 		public void InOverloadIsTheOnlyCandidateWithInAtTheCall()
 		{

ICSharpCode.Decompiler.Tests/TestCases/Pretty/FirstClassSpanTypes.cs

@@ -104,6 +104,23 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 		{
 		}
 
+		public static void RefSpanOrByValue(ref ReadOnlySpan<int> s)
+		{
+		}
+
+		public static void RefSpanOrByValue(ReadOnlySpan<int> s)
+		{
+		}
+
+		public static void OutSpanOrByValue(out ReadOnlySpan<int> s)
+		{
+			s = default(ReadOnlySpan<int>);
+		}
+
+		public static void OutSpanOrByValue(ReadOnlySpan<int> s)
+		{
+		}
+
 		public static void GenericArrayOrReadOnlySpan<T>(T[] a)
 		{
 		}
@@ -159,12 +176,25 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 			ReadOnlySpanOfObjectOrString(strArr);
 			StringOrReadOnlySpanChar(str);
 			ParamsArrayOrParamsReadOnlySpan(arr);
+			ParamsArrayOrParamsReadOnlySpan(1, 2, 3);
 			GenericArrayOrReadOnlySpan(arr);
 			InferFromReadOnlySpan(arr);
 			InferFromReadOnlySpan(span);
 			arr.ExtensionOnReadOnlySpan();
 		}
 
+		public static void CallRefOutOrByValue(int[] arr)
+		{
+			// A span conversion never binds a ref or out parameter: without the keyword the
+			// by-value overload wins, with the keyword only the ref/out overload is
+			// applicable and the keyword must survive decompilation.
+			ReadOnlySpan<int> s = arr;
+			RefSpanOrByValue(arr);
+			RefSpanOrByValue(ref s);
+			OutSpanOrByValue(arr);
+			OutSpanOrByValue(out s);
+		}
+
 		public static void CallLosersWithExplicitConversions(int[] arr, string str)
 		{
 			ArrayOrReadOnlySpan((ReadOnlySpan<int>)arr);