Fix #3894: keep lambda parameter typing consistent

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

@@ -174,5 +174,18 @@ private static Action<T> MakeAction<T>(T template)
 		{
 			return null;
 		}
+#if ROSLYN
+		private static TValue GetOrCreate<TKey, TValue>(TKey key, Func<TKey, int, TValue> factory)
+		{
+			return factory(key, 1);
+		}
+
+		private static int MixedLambdaParameters()
+		{
+			return GetOrCreate(new {
+				Value = 1
+			}, (item, context) => item.Value + context);
+		}
+#endif
 	}
 }

ICSharpCode.Decompiler/CSharp/ExpressionBuilder.cs

@@ -2669,6 +2669,8 @@ IType GetTaskType(IType resultType)
 		IEnumerable<ParameterDeclaration> MakeParameters(IReadOnlyList<IParameter> parameters, ILFunction function)
 		{
 			var variables = function.Variables.Where(v => v.Kind == VariableKind.Parameter).ToDictionary(v => v.Index!.Value);
+			var result = new List<ParameterDeclaration>(parameters.Count);
+			bool anyAnonymousType = false;
 			int i = 0;
 			foreach (var parameter in parameters)
 			{
@@ -2684,10 +2686,21 @@ IEnumerable<ParameterDeclaration> MakeParameters(IReadOnlyList<IParameter> param
 					pd.Name = "P_" + i;
 				}
 				if (settings.AnonymousTypes && parameter.Type.ContainsAnonymousType())
-					pd.Type = null;
-				yield return pd;
+					anyAnonymousType = true;
+				result.Add(pd);
 				i++;
 			}
+			// An anonymous type cannot be named, so a lambda with such a parameter must be implicitly typed.
+			// C# also requires all lambda parameters to use the same form (CS0748). Drop every type when the
+			// remaining parameter syntax permits it; otherwise keep the converted declarations as a
+			// best-effort fallback for an unrepresentable signature.
+			if (anyAnonymousType && result.All(p => p.ParameterModifier == ReferenceKind.None
+				&& !p.IsParams && !p.IsScopedRef && p.Attributes.Count == 0 && p.DefaultExpression is null))
+			{
+				foreach (var pd in result)
+					pd.Type = null;
+			}
+			return result;
 		}
 
 		protected internal override TranslatedExpression VisitBlockContainer(BlockContainer container, TranslationContext context)