Keep a discarded method-group conversion out of the delegate cache

CachedDelegateInitializationWithField only fired when the instruction
after the caching `if` read the cache field exactly once. A discarded
conversion -- `_ = (Action<int>)M;` -- reads it zero times: Roslyn emits
the null check and the store, and nothing else. The transform bailed out,
so the `<>O` cache class and its `<0>__M` field survived into the output
under names no C# compiler will accept.

Zero usages is now handled: the `if` is replaced by the delegate
construction it guarded, which is what the source expressed, and the
caching disappears with the field. The rest of the method is checked for
loads of the same field first, since only then is this `if` the sole
initialization.

Closes #3965

ICSharpCode.Decompiler.Tests/PrettyTestRunner.cs

@@ -926,6 +926,12 @@ namespace ICSharpCode.Decompiler.Tests
 			await RunForLibrary(cscOptions: cscOptions);
 		}
 
+		[Test]
+		public async Task Issue3965([ValueSource(nameof(roslyn4OrNewerOptions))] CompilerOptions cscOptions)
+		{
+			await RunForLibrary(cscOptions: cscOptions);
+		}
+
 		[Test]
 		public async Task AssemblyCustomAttributes([ValueSource(nameof(defaultOptions))] CompilerOptions cscOptions)
 		{

ICSharpCode.Decompiler.Tests/TestCases/Pretty/Issue3965.cs

@@ -0,0 +1,29 @@
+using System;
+
+namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
+{
+	internal class Issue3965
+	{
+		private static void M(int x)
+		{
+		}
+
+		private static void Use(Action<int> action)
+		{
+		}
+
+		public static void DiscardedMethodGroupConversion()
+		{
+#if EXPECTED_OUTPUT
+			new Action<int>(M);
+#else
+			_ = (Action<int>)M;
+#endif
+		}
+
+		public static void UsedMethodGroupConversion()
+		{
+			Use(M);
+		}
+	}
+}

ICSharpCode.Decompiler/IL/Transforms/CachedDelegateInitialization.cs

@@ -37,7 +37,6 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 				{
 					if (CachedDelegateInitializationWithField(inst))
 					{
-						block.Instructions.RemoveAt(i);
 						context.IndexOfFirstAlreadyTransformedInstruction = block.Instructions.Count;
 						continue;
 					}
@@ -95,13 +94,25 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 			if (!DelegateConstruction.MatchDelegateConstruction(value.UnwrapConv(ConversionKind.Invalid) as NewObj, out _, out _, out _, true))
 				return false;
 			var nextInstruction = inst.Parent.Children.ElementAtOrDefault(inst.ChildIndex + 1);
-			if (nextInstruction == null)
-				return false;
-			var usages = nextInstruction.Descendants.Where(i => i.MatchLdsFld(field)).ToArray();
+			var usages = nextInstruction?.Descendants.Where(i => i.MatchLdsFld(field)).ToArray() ?? [];
+			if (usages.Length == 0)
+			{
+				// A discarded method-group conversion ("_ = (Action)M;") caches the
+				// delegate without ever reading the cache back. Keep the conversion,
+				// which still allocates, and drop the caching around it -- but only
+				// once the rest of the method is known not to read the field either.
+				if (context.Function.Descendants.Any(i => i != left && i.MatchLdsFld(field)))
+					return false;
+				context.Step("CachedDelegateInitializationWithField (unused)", inst);
+				inst.ReplaceWith(value);
+				context.EndStep(value);
+				return true;
+			}
 			if (usages.Length != 1)
 				return false;
 			context.Step("CachedDelegateInitializationWithField", inst);
 			usages[0].ReplaceWith(value);
+			((Block)inst.Parent).Instructions.RemoveAt(inst.ChildIndex);
 			context.EndStep(value);
 			return true;
 		}