Fix #2964: Better error message when a method only contains a single ret instruction.

ICSharpCode.Decompiler.Tests/TestCases/ILPretty/EmptyBodies.cs

@@ -5,7 +5,7 @@ internal class EmptyBodies
 	}
 	public static int RetInt()
 	{
-		return (int)/*Error near IL_0001: Stack underflow*/;
+		/*Error: Method body consists only of 'ret', but nothing is being returned. Decompiled assembly might be a reference assembly.*/;
 	}
 	public static void Nop()
 	{

ICSharpCode.Decompiler/IL/BlockBuilder.cs

@@ -129,15 +129,6 @@ namespace ICSharpCode.Decompiler.IL
 		{
 			CreateContainerStructure();
 			mainContainer.SetILRange(new Interval(0, body.GetCodeSize()));
-			if (!basicBlocks.Any())
-			{
-				mainContainer.Blocks.Add(new Block {
-					Instructions = {
-						new InvalidBranch("Empty body found. Decompiled assembly might be a reference assembly.")
-					}
-				});
-				return;
-			}
 
 			currentContainer = mainContainer;
 			foreach (var block in basicBlocks.OrderBy(b => b.StartILOffset))

ICSharpCode.Decompiler/IL/ILReader.cs

@@ -457,6 +457,13 @@ namespace ICSharpCode.Decompiler.IL
 		{
 			reader.Reset();
 			StoreStackForOffset(0, ImmutableStack<ILVariable>.Empty);
+			if (reader.Length == 0)
+			{
+				blocksByOffset[0].Block.Instructions.Add(
+					new InvalidBranch("Empty body found. Decompiled assembly might be a reference assembly.")
+				);
+				return;
+			}
 			ILParser.SetBranchTargets(ref reader, isBranchTarget);
 			PrepareBranchTargetsAndStacksForExceptionHandlers();
 
@@ -1480,9 +1487,18 @@ namespace ICSharpCode.Decompiler.IL
 		private ILInstruction Return()
 		{
 			if (methodReturnStackType == StackType.Void)
+			{
 				return new IL.Leave(mainContainer);
+			}
+			else if (currentInstructionStart == 0)
+			{
+				Debug.Assert(expressionStack.Count == 0 && currentStack.IsEmpty);
+				return new InvalidBranch("Method body consists only of 'ret', but nothing is being returned. Decompiled assembly might be a reference assembly.");
+			}
 			else
+			{
 				return new IL.Leave(mainContainer, Pop(methodReturnStackType));
+			}
 		}
 
 		private ILInstruction DecodeLdstr()