Fix extension method calls on `this in`.

ICSharpCode.Decompiler/CSharp/Resolver/CSharpResolver.cs

@@ -2158,12 +2158,17 @@ namespace ICSharpCode.Decompiler.CSharp.Resolver
 			if (method.Parameters.Count == 0)
 				return false;
 			IType thisParameterType = method.Parameters[0].Type;
+			if (thisParameterType.Kind == TypeKind.ByReference)
+			{
+				// extension method with `this in` or `this ref`
+				thisParameterType = ((ByReferenceType)thisParameterType).ElementType;
+			}
 			if (useTypeInference && method.TypeParameters.Count > 0)
 			{
 				// We need to infer type arguments from targetType:
 				TypeInference ti = new TypeInference(compilation, conversions);
 				ResolveResult[] arguments = { new ResolveResult(targetType) };
-				IType[] parameterTypes = { method.Parameters[0].Type };
+				IType[] parameterTypes = { thisParameterType };
 				var inferredTypes = ti.InferTypeArguments(method.TypeParameters, arguments, parameterTypes, out _);
 				var substitution = new TypeParameterSubstitution(null, inferredTypes);
 				// Validate that the types that could be inferred (aren't unknown) satisfy the constraints:

ICSharpCode.Decompiler/CSharp/Resolver/MethodGroupResolveResult.cs

@@ -271,6 +271,7 @@ namespace ICSharpCode.Decompiler.CSharp.Resolver
 					extOr.AllowOptionalParameters = allowOptionalParameters;
 					extOr.IsExtensionMethodInvocation = true;
 					extOr.CheckForOverflow = checkForOverflow;
+					extOr.AllowImplicitIn = allowImplicitIn;
 
 					foreach (var g in extensionMethods)
 					{

ICSharpCode.Decompiler/CSharp/Resolver/OverloadResolution.cs

@@ -669,8 +669,11 @@ namespace ICSharpCode.Decompiler.CSharp.Resolver
 				}
 				else
 				{
-					if (paramRefKind == ReferenceKind.In && AllowImplicitIn
-						&& candidate.ParameterTypes[parameterIndex].SkipModifiers() is ByReferenceType brt)
+					// AllowImplicitIn: `in` parameters can be filled implicitly without `in` DirectionExpression
+					// IsExtensionMethodInvocation: `this ref` and `this in` parameters can be filled implicitly
+					if (((paramRefKind == ReferenceKind.In && AllowImplicitIn)
+						|| (IsExtensionMethodInvocation && parameterIndex == 0 && (paramRefKind == ReferenceKind.In || paramRefKind == ReferenceKind.Ref))
+						) && candidate.ParameterTypes[parameterIndex].SkipModifiers() is ByReferenceType brt)
 					{
 						// Treat the parameter as if it was not declared "in" for the following steps
 						// (applicability + better function member)

ICSharpCode.Decompiler/CSharp/Transforms/IntroduceExtensionMethods.cs

@@ -16,7 +16,6 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
-using System;
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Linq;
@@ -181,6 +180,10 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			{
 				target = new ConversionResolveResult(method.Parameters[0].Type, crr, Conversion.NullLiteralConversion);
 			}
+			else if (target is ByReferenceResolveResult brrr)
+			{
+				target = brrr.ElementResult;
+			}
 			ResolveResult[] args = new ResolveResult[invocationExpression.Arguments.Count - 1];
 			string[] argNames = null;
 			int pos = 0;