Add an async/await pattern test suite and fix the await operand conversions it found

ICSharpCode.Decompiler.Tests/CorrectnessTestRunner.cs

@@ -398,6 +398,12 @@ public async Task Async([ValueSource(nameof(noMonoOptions))] CompilerOptions opt
 			await RunCS(options: options);
 		}
 
+		[Test]
+		public async Task AsyncAwaitPatterns([ValueSource(nameof(noMonoOptions))] CompilerOptions options)
+		{
+			await RunCS(options: options);
+		}
+
 		[Test]
 		public async Task LINQRaytracer([ValueSource(nameof(defaultOptions))] CompilerOptions options)
 		{

ICSharpCode.Decompiler.Tests/PrettyTestRunner.cs

@@ -560,6 +560,12 @@ public async Task AsyncStreams([ValueSource(nameof(roslyn3OrNewerOptions))] Comp
 			await RunForLibrary(cscOptions: cscOptions);
 		}
 
+		[Test]
+		public async Task AsyncAwaitPatterns([ValueSource(nameof(defaultOptions))] CompilerOptions cscOptions)
+		{
+			await RunForLibrary(cscOptions: cscOptions);
+		}
+
 		[Test]
 		public async Task AsyncUsing([ValueSource(nameof(roslyn3OrNewerOptions))] CompilerOptions cscOptions)
 		{

ICSharpCode.Decompiler.Tests/TestCases/Correctness/AsyncAwaitPatterns.cs

@@ -0,0 +1,228 @@
+// Copyright (c) 2026 Siegfried Pammer
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy of this
+// software and associated documentation files (the "Software"), to deal in the Software
+// without restriction, including without limitation the rights to use, copy, modify, merge,
+// publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons
+// to whom the Software is furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in all copies or
+// substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED,
+// INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR
+// PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE
+// FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
+// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+// DEALINGS IN THE SOFTWARE.
+
+#pragma warning disable 1998
+using System;
+using System.Collections.Generic;
+using System.Runtime.CompilerServices;
+using System.Threading.Tasks;
+
+namespace ICSharpCode.Decompiler.Tests.TestCases.Correctness
+{
+	// The Pretty fixture of the same name pins how awaits are printed; this one pins what they
+	// have to mean: copy semantics of struct awaitables, and the evaluation order around the
+	// suspension point.
+	public class AsyncAwaitPatterns
+	{
+		public struct CountingAwaitable
+		{
+			public int Counter;
+
+			public TaskAwaiter<int> GetAwaiter()
+			{
+				Counter++;
+				Console.WriteLine("  GetAwaiter, Counter is now " + Counter);
+				return Task.FromResult(0).GetAwaiter();
+			}
+		}
+
+		public class Holder
+		{
+			public CountingAwaitable Mutable;
+			public readonly CountingAwaitable ReadOnly;
+
+			public CountingAwaitable Property {
+				get { return Mutable; }
+			}
+		}
+
+		private int[] array = new int[4];
+		private int index;
+		private int field;
+
+		public static void Main()
+		{
+			new AsyncAwaitPatterns().Run().Wait();
+		}
+
+		public async Task Run()
+		{
+			await MutableStructField();
+			await ReadOnlyStructField();
+			await StructProperty();
+			await CompoundAssignmentToArrayElement();
+			await AssignmentAfterAwait();
+			await ArgumentEvaluationOrder();
+			await RefArgumentEvaluationOrder();
+			await AwaitInLoop();
+			await AwaitInTernary(true);
+			await AwaitInTernary(false);
+#if CS60
+			await AwaitInCatchAndFinally();
+#endif
+			Console.WriteLine("done");
+		}
+
+		private Task<int> Value(int v)
+		{
+			Console.WriteLine("  Value(" + v + ")");
+			return Task.FromResult(v);
+		}
+
+		private int Index(string tag)
+		{
+			Console.WriteLine("  Index(" + tag + ") -> " + index);
+			return index;
+		}
+
+		private int[] Array(string tag)
+		{
+			Console.WriteLine("  Array(" + tag + ")");
+			return array;
+		}
+
+		private int Side()
+		{
+			Console.WriteLine("  Side()");
+			return 100;
+		}
+
+		private static string Combine(int a, int b, int c)
+		{
+			return a + "/" + b + "/" + c;
+		}
+
+		private static void AddTo(ref int slot, int addend)
+		{
+			Console.WriteLine("  AddTo(" + slot + ", " + addend + ")");
+			slot += addend;
+		}
+
+		// GetAwaiter is called on the field itself, so its mutation sticks.
+		public async Task MutableStructField()
+		{
+			Console.WriteLine("MutableStructField");
+			Holder holder = new Holder();
+			await holder.Mutable;
+			await holder.Mutable;
+			Console.WriteLine("  Counter = " + holder.Mutable.Counter);
+		}
+
+		// A readonly field is defensively copied, so the mutation is discarded.
+		public async Task ReadOnlyStructField()
+		{
+			Console.WriteLine("ReadOnlyStructField");
+			Holder holder = new Holder();
+			await holder.ReadOnly;
+			await holder.ReadOnly;
+			Console.WriteLine("  Counter = " + holder.ReadOnly.Counter);
+		}
+
+		// A property returns a copy, so the mutation is discarded as well.
+		public async Task StructProperty()
+		{
+			Console.WriteLine("StructProperty");
+			Holder holder = new Holder();
+			await holder.Property;
+			await holder.Property;
+			Console.WriteLine("  Counter = " + holder.Mutable.Counter);
+		}
+
+		// Target and index are evaluated before the await, not after it.
+		public async Task CompoundAssignmentToArrayElement()
+		{
+			Console.WriteLine("CompoundAssignmentToArrayElement");
+			array = new int[4];
+			index = 0;
+			Array("lhs")[Index("lhs")] += await Value(5);
+			index = 1;
+			Console.WriteLine("  array = " + string.Join(",", array));
+		}
+
+		public async Task AssignmentAfterAwait()
+		{
+			Console.WriteLine("AssignmentAfterAwait");
+			array = new int[4];
+			index = 2;
+			int[] target = Array("target");
+			int i = Index("i");
+			index = 3;
+			target[i] = await Value(7);
+			Console.WriteLine("  array = " + string.Join(",", array));
+		}
+
+		public async Task ArgumentEvaluationOrder()
+		{
+			Console.WriteLine("ArgumentEvaluationOrder");
+			Console.WriteLine("  " + Combine(await Value(1), Side(), await Value(2)));
+		}
+
+		public async Task RefArgumentEvaluationOrder()
+		{
+			Console.WriteLine("RefArgumentEvaluationOrder");
+			field = 0;
+			AddTo(ref field, await Value(6));
+			Console.WriteLine("  field = " + field);
+		}
+
+		public async Task AwaitInLoop()
+		{
+			Console.WriteLine("AwaitInLoop");
+			for (int i = 0; i < 4; i++)
+			{
+				if (i == 1)
+				{
+					continue;
+				}
+				if (i == 3)
+				{
+					break;
+				}
+				Console.WriteLine("  loop " + await Value(i));
+			}
+		}
+
+		public async Task AwaitInTernary(bool condition)
+		{
+			Console.WriteLine("AwaitInTernary(" + condition + ")");
+			Console.WriteLine("  " + (condition ? await Value(1) : await Value(2)));
+		}
+
+#if CS60
+		public async Task AwaitInCatchAndFinally()
+		{
+			Console.WriteLine("AwaitInCatchAndFinally");
+			try
+			{
+				await Value(1);
+				throw new InvalidOperationException("boom");
+			}
+			catch (InvalidOperationException ex)
+			{
+				Console.WriteLine("  caught " + ex.Message);
+				await Value(2);
+			}
+			finally
+			{
+				Console.WriteLine("  finally");
+				await Value(3);
+			}
+		}
+#endif
+	}
+}