Fix #3180: Spell out type arguments of a null-conditional extension call

When the extension method call cannot be written without type arguments
(another extension method with the same name and a different arity is
found first), the call falls back to the static form. For 'a?.M()' that
fallback produced 'Enumerable.Count(s?)', which is not valid C#. Writing
the type arguments excludes the extension methods of a different arity,
so for a null-conditional target the type arguments are tried before
giving up.

ICSharpCode.Decompiler/CSharp/CallBuilder.cs

@@ -1318,8 +1318,8 @@ namespace ICSharpCode.Decompiler.CSharp
 				return false;
 
 			var firstArgument = argumentList.Arguments[0];
-			bool writeTypeArguments = (transform & ReferenceTransformation.RequireTypeArguments) != 0
-				&& (!settings.AnonymousTypes || !method.TypeArguments.Any(a => a.ContainsAnonymousType()));
+			bool canWriteTypeArguments = !settings.AnonymousTypes || !method.TypeArguments.Any(a => a.ContainsAnonymousType());
+			bool writeTypeArguments = (transform & ReferenceTransformation.RequireTypeArguments) != 0 && canWriteTypeArguments;
 			IType[] typeArguments = writeTypeArguments ? method.TypeArguments.ToArray() : Empty<IType>.Array;
 
 			var directionExpression = firstArgument.Expression as DirectionExpression;
@@ -1342,10 +1342,20 @@ namespace ICSharpCode.Decompiler.CSharp
 			string[]? remainingNames = argumentNames?.Take(actualArgumentCount).Skip(1).ToArray();
 			if (remainingNames != null && remainingNames.All(name => name == null))
 				remainingNames = null;
-			if (!resolver.CanTransformToExtensionMethodCall(method, typeArguments, target,
-				argumentList.GetArgumentResolveResults(skipCount: 1).ToArray(), remainingNames))
+			var remainingArguments = argumentList.GetArgumentResolveResults(skipCount: 1).ToArray();
+			if (!resolver.CanTransformToExtensionMethodCall(method, typeArguments, target, remainingArguments, remainingNames))
 			{
-				return false;
+				// 'a?.M()' has no static call form to fall back to. Spelled-out type arguments rule out
+				// extension methods of a different arity, which NullPropagationTransform relies on.
+				if (writeTypeArguments || !canWriteTypeArguments || method.TypeArguments.Count == 0
+					|| !IsNullConditional(firstArgument.Expression))
+				{
+					return false;
+				}
+				writeTypeArguments = true;
+				typeArguments = method.TypeArguments.ToArray();
+				if (!resolver.CanTransformToExtensionMethodCall(method, typeArguments, target, remainingArguments, remainingNames))
+					return false;
 			}
 
 			Expression targetExpression;