Prevent argument count mismatch

The expression splitting logic is a little hacky when it comes to
assignments to property or indexer setters: first, a split operation
is performed as if we were looking at a property or indexer *getter*;
the `InvocationShape` receives the correct method, but is one argument
short. Once that first `InvocationShape` has been built, we can create
a second, patched and fully correct version that includes the assign-
ment (RHS) argument.

This commit relaxes validation rules for the first `InvocationShape`
(i.e. the one with the potential argument count mismatch).

src/Moq/ExpressionExtensions.cs

@@ -186,7 +186,7 @@ void Split(Expression e, out Expression r /* remainder */, out InvocationShape p
 							property = ((IndexExpression)lhs.Expression.Body).Indexer;
 						}
 						var parameter = Expression.Parameter(r.Type, r is ParameterExpression ope ? ope.Name : ParameterName);
-						var arguments = new Expression[lhs.Arguments.Count + 1];
+						var arguments = new Expression[lhs.Method.GetParameters().Length];
 						for (var ai = 0; ai < arguments.Length - 1; ++ai)
 						{
 							arguments[ai] = lhs.Arguments[ai];
@@ -254,13 +254,14 @@ void Split(Expression e, out Expression r /* remainder */, out InvocationShape p
 						r = indexExpression.Object;
 						var parameter = Expression.Parameter(r.Type, r is ParameterExpression ope ? ope.Name : ParameterName);
 						var indexer = indexExpression.Indexer;
-						var arguments = indexExpression.Arguments;
+						var canRead = indexer.CanRead(out var getter);
 						p = new InvocationShape(
 									expression: Expression.Lambda(
-										Expression.MakeIndex(parameter, indexer, arguments),
+										Expression.MakeIndex(parameter, indexer, indexExpression.Arguments),
 										parameter),
-									method: indexer.GetGetMethod(true),
-									arguments);
+									method: canRead ? getter : indexer.GetSetMethod(true),
+									arguments: indexExpression.Arguments,
+									skipMatcherInitialization: !canRead);
 						return;
 					}
 

src/Moq/InvocationShape.cs

@@ -72,7 +72,7 @@ public static InvocationShape CreateFrom(Invocation invocation)
 #endif
 		private readonly bool exactGenericTypeArguments;
 
-		public InvocationShape(LambdaExpression expression, MethodInfo method, IReadOnlyList<Expression> arguments = null, bool exactGenericTypeArguments = false)
+		public InvocationShape(LambdaExpression expression, MethodInfo method, IReadOnlyList<Expression> arguments = null, bool exactGenericTypeArguments = false, bool skipMatcherInitialization = false)
 		{
 			Debug.Assert(expression != null);
 			Debug.Assert(method != null);
@@ -82,14 +82,14 @@ public InvocationShape(LambdaExpression expression, MethodInfo method, IReadOnly
 
 			this.Expression = expression;
 			this.Method = method;
-			if (arguments != null)
+			if (arguments != null && !skipMatcherInitialization)
 			{
 				(this.argumentMatchers, this.Arguments) = MatcherFactory.CreateMatchers(arguments, method.GetParameters());
 			}
 			else
 			{
 				this.argumentMatchers = noArgumentMatchers;
-				this.Arguments = noArguments;
+				this.Arguments = arguments ?? noArguments;
 			}
 
 			this.exactGenericTypeArguments = exactGenericTypeArguments;