Fix ExpectedResultType of main BlockContainer.

ICSharpCode.Decompiler/IL/ControlFlow/AsyncAwaitDecompiler.cs

@@ -95,6 +95,7 @@ namespace ICSharpCode.Decompiler.IL.ControlFlow
 			TranslateCachedFieldsToLocals();
 
 			FinalizeInlineMoveNext(function);
+			((BlockContainer)function.Body).ExpectedResultType = underlyingReturnType.GetStackType();
 
 			// Re-run control flow simplification over the newly constructed set of gotos,
 			// and inlining because TranslateFieldsToLocalAccess() might have opened up new inlining opportunities.

ICSharpCode.Decompiler/IL/ILReader.cs

@@ -83,7 +83,7 @@ namespace ICSharpCode.Decompiler.IL
 					v.HasInitialValue = true;
 				}
 			}
-			mainContainer = new BlockContainer();
+			mainContainer = new BlockContainer(methodReturnStackType);
 			this.instructionBuilder = new List<ILInstruction>();
 			this.isBranchTarget = new BitArray(body.CodeSize);
 			this.stackByOffset = new Dictionary<int, ImmutableStack<ILVariable>>();

ICSharpCode.Decompiler/IL/Instructions/BlockContainer.cs

@@ -39,7 +39,7 @@ namespace ICSharpCode.Decompiler.IL
 		public static readonly SlotInfo BlockSlot = new SlotInfo("Block", isCollection: true);
 		public readonly InstructionCollection<Block> Blocks;
 
-		public StackType ExpectedResultType { get; }
+		public StackType ExpectedResultType { get; set; }
 		
 		/// <summary>
 		/// Gets the number of 'leave' instructions that target this BlockContainer.

ICSharpCode.Decompiler/IL/Instructions/Leave.cs

@@ -97,6 +97,7 @@ namespace ICSharpCode.Decompiler.IL
 		{
 			base.CheckInvariant(phase);
 			Debug.Assert(phase <= ILPhase.InILReader || this.IsDescendantOf(targetContainer));
+			Debug.Assert(phase <= ILPhase.InILReader || value.ResultType == targetContainer.ResultType);
 		}
 		
 		public override void WriteTo(ITextOutput output)