#1675: Fix converting static method to method group

ICSharpCode.Decompiler.Tests/TestCases/Ugly/NoExtensionMethods.Expected.cs

@@ -19,5 +19,15 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Ugly
 		{
 			return value;
 		}
+
+		internal static Func<int, int> ExtensionMethodAsStaticFunc()
+		{
+			return Return;
+		}
+
+		internal static Func<object> ExtensionMethodBoundToNull()
+		{
+			return new Func<object>(null, __ldftn(Return));
+		}
 	}
 }

ICSharpCode.Decompiler.Tests/TestCases/Ugly/NoExtensionMethods.cs

@@ -13,5 +13,15 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Ugly
 		{
 			return value;
 		}
+
+		internal static Func<int, int> ExtensionMethodAsStaticFunc()
+		{
+			return Return;
+		}
+
+		internal static Func<object> ExtensionMethodBoundToNull()
+		{
+			return ((object)null).Return;
+		}
 	}
 }

ICSharpCode.Decompiler.Tests/TestCases/Ugly/NoExtensionMethods.opt.roslyn.il

@@ -61,6 +61,30 @@
     IL_0001:  ret
   } // end of method NoExtensionMethods::Return
 
+  .method assembly hidebysig static class [mscorlib]System.Func`2<int32,int32> 
+          ExtensionMethodAsStaticFunc() cil managed
+  {
+    // Code size       13 (0xd)
+    .maxstack  8
+    IL_0000:  ldnull
+    IL_0001:  ldftn      !!0 ICSharpCode.Decompiler.Tests.TestCases.Ugly.NoExtensionMethods::Return<int32>(!!0)
+    IL_0007:  newobj     instance void class [mscorlib]System.Func`2<int32,int32>::.ctor(object,
+                                                                                         native int)
+    IL_000c:  ret
+  } // end of method NoExtensionMethods::ExtensionMethodAsStaticFunc
+
+  .method assembly hidebysig static class [mscorlib]System.Func`1<object> 
+          ExtensionMethodBoundToNull() cil managed
+  {
+    // Code size       13 (0xd)
+    .maxstack  8
+    IL_0000:  ldnull
+    IL_0001:  ldftn      !!0 ICSharpCode.Decompiler.Tests.TestCases.Ugly.NoExtensionMethods::Return<object>(!!0)
+    IL_0007:  newobj     instance void class [mscorlib]System.Func`1<object>::.ctor(object,
+                                                                                    native int)
+    IL_000c:  ret
+  } // end of method NoExtensionMethods::ExtensionMethodBoundToNull
+
 } // end of class ICSharpCode.Decompiler.Tests.TestCases.Ugly.NoExtensionMethods
 
 

ICSharpCode.Decompiler.Tests/TestCases/Ugly/NoExtensionMethods.roslyn.il

@@ -73,6 +73,42 @@
     IL_0006:  ret
   } // end of method NoExtensionMethods::Return
 
+  .method assembly hidebysig static class [mscorlib]System.Func`2<int32,int32> 
+          ExtensionMethodAsStaticFunc() cil managed
+  {
+    // Code size       18 (0x12)
+    .maxstack  2
+    .locals init (class [mscorlib]System.Func`2<int32,int32> V_0)
+    IL_0000:  nop
+    IL_0001:  ldnull
+    IL_0002:  ldftn      !!0 ICSharpCode.Decompiler.Tests.TestCases.Ugly.NoExtensionMethods::Return<int32>(!!0)
+    IL_0008:  newobj     instance void class [mscorlib]System.Func`2<int32,int32>::.ctor(object,
+                                                                                         native int)
+    IL_000d:  stloc.0
+    IL_000e:  br.s       IL_0010
+
+    IL_0010:  ldloc.0
+    IL_0011:  ret
+  } // end of method NoExtensionMethods::ExtensionMethodAsStaticFunc
+
+  .method assembly hidebysig static class [mscorlib]System.Func`1<object> 
+          ExtensionMethodBoundToNull() cil managed
+  {
+    // Code size       18 (0x12)
+    .maxstack  2
+    .locals init (class [mscorlib]System.Func`1<object> V_0)
+    IL_0000:  nop
+    IL_0001:  ldnull
+    IL_0002:  ldftn      !!0 ICSharpCode.Decompiler.Tests.TestCases.Ugly.NoExtensionMethods::Return<object>(!!0)
+    IL_0008:  newobj     instance void class [mscorlib]System.Func`1<object>::.ctor(object,
+                                                                                    native int)
+    IL_000d:  stloc.0
+    IL_000e:  br.s       IL_0010
+
+    IL_0010:  ldloc.0
+    IL_0011:  ret
+  } // end of method NoExtensionMethods::ExtensionMethodBoundToNull
+
 } // end of class ICSharpCode.Decompiler.Tests.TestCases.Ugly.NoExtensionMethods
 
 

ICSharpCode.Decompiler/CSharp/CallBuilder.cs

@@ -1235,12 +1235,38 @@ namespace ICSharpCode.Decompiler.CSharp
 				default:
 					throw new ArgumentException($"Unknown instruction type: {func.OpCode}");
 			}
-			if (method.IsStatic  && !method.IsExtensionMethod) {
+			if (CanUseDelegateConstruction(method, thisArg, inst.Method.DeclaringType.GetDelegateInvokeMethod())) {
+				return HandleDelegateConstruction(inst.Method.DeclaringType, method, expectedTargetDetails, thisArg, inst);
+			} else {
 				var argumentList = BuildArgumentList(expectedTargetDetails, null, inst.Method,
 					0, inst.Arguments, null);
 				return HandleConstructorCall(new ExpectedTargetDetails { CallOpCode = OpCode.NewObj }, null, inst.Method, argumentList).WithILInstruction(inst);
 			}
-			return HandleDelegateConstruction(inst.Method.DeclaringType, method, expectedTargetDetails, thisArg, inst);
+		}
+
+		private bool CanUseDelegateConstruction(IMethod targetMethod, ILInstruction thisArg, IMethod invokeMethod)
+		{
+			if (targetMethod.IsStatic) {
+				// If the invoke method is known, we can compare the parameter counts to figure out whether the
+				// delegate is static or binds the first argument
+				if (invokeMethod != null) {
+					if (invokeMethod.Parameters.Count == targetMethod.Parameters.Count) {
+						return thisArg.MatchLdNull();
+					} else if (targetMethod.IsExtensionMethod && invokeMethod.Parameters.Count == targetMethod.Parameters.Count - 1) {
+						return true;
+					} else {
+						return false;
+					}
+				} else {
+					// delegate type unknown:
+					return thisArg.MatchLdNull() || targetMethod.IsExtensionMethod;
+				}
+			} else {
+				// targetMethod is instance method
+				if (invokeMethod != null && invokeMethod.Parameters.Count != targetMethod.Parameters.Count)
+					return false;
+				return true;
+			}
 		}
 
 		internal TranslatedExpression Build(LdVirtDelegate inst)