Handle multiple calling conventions at once

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

@@ -129,6 +129,32 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 			}
 		}
 
+		public unsafe delegate* unmanaged[Cdecl, Fastcall]<void> AddressOfLocalFunction_CDeclAndFastcall()
+		{
+			return &LocalFunction;
+
+			[UnmanagedCallersOnly(CallConvs = new Type[] {
+				typeof(CallConvCdecl),
+				typeof(CallConvFastcall)
+			})]
+			static void LocalFunction()
+			{
+			}
+		}
+
+		public unsafe delegate* unmanaged[Fastcall, Cdecl]<void> AddressOfLocalFunction_FastcallAndCDecl()
+		{
+			return &LocalFunction;
+
+			[UnmanagedCallersOnly(CallConvs = new Type[] {
+				typeof(CallConvFastcall),
+				typeof(CallConvCdecl)
+			})]
+			static void LocalFunction()
+			{
+			}
+		}
+
 #if NET60
 		public unsafe delegate* unmanaged[Cdecl, SuppressGCTransition]<void> AddressOfLocalFunction_CDeclAndSuppressGCTransition()
 		{

ICSharpCode.Decompiler/CSharp/ExpressionBuilder.cs

@@ -4611,7 +4611,7 @@ namespace ICSharpCode.Decompiler.CSharp
 					var builder = ImmutableArray.CreateBuilder<IType>(array.Length);
 					foreach (var type in array.Select(a => a.Value).OfType<IType>())
 					{
-						SignatureCallingConvention? foundCallingConvention = type.Namespace is not "System.Runtime.CompilerServices" ? null : type.Name switch {
+						SignatureCallingConvention? foundCallingConvention = type.Namespace is not "System.Runtime.CompilerServices" || callingConvention != SignatureCallingConvention.Unmanaged ? null : type.Name switch {
 							"CallConvCdecl" => SignatureCallingConvention.CDecl,
 							"CallConvFastcall" => SignatureCallingConvention.FastCall,
 							"CallConvStdcall" => SignatureCallingConvention.StdCall,

ICSharpCode.Decompiler/TypeSystem/FunctionPointerType.cs

@@ -46,6 +46,8 @@ namespace ICSharpCode.Decompiler.TypeSystem
 					&& modReturn.Modifier.Namespace == "System.Runtime.CompilerServices")
 				{
 					returnType = modReturn.ElementType;
+					if (callingConvention == SignatureCallingConvention.Unmanaged)
+					{
 						switch (modReturn.Modifier.Name)
 						{
 							case "CallConvCdecl":
@@ -66,6 +68,11 @@ namespace ICSharpCode.Decompiler.TypeSystem
 						}
 					}
 					else
+					{
+						customCallConvs.Add(modReturn.Modifier);
+					}
+				}
+				else
 				{
 					break;
 				}