fix: extension method parameters

src/Riok.Mapperly/Emit/Syntax/SyntaxFactoryHelper.Invocation.cs

@@ -84,7 +84,7 @@ public static ParameterListSyntax ParameterList(bool extensionMethod, params Met
             .WhereNotNull()
             .DistinctBy(x => x.Ordinal)
             .OrderBy(x => x.Ordinal)
-            .Select(p => Parameter(extensionMethod, p));
+            .Select((p, i) => Parameter(extensionMethod && i == 0, p));
         return SyntaxFactory.ParameterList(CommaSeparatedList(parameterSyntaxes));
     }
 

test/Riok.Mapperly.Tests/Mapping/ExtensionMethodTest.cs

@@ -0,0 +1,29 @@
+namespace Riok.Mapperly.Tests.Mapping;
+
+[UsesVerify]
+public class ExtensionMethodTest
+{
+    [Fact]
+    public Task ExtensionMapMethodShouldWork()
+    {
+        var source = TestSourceBuilder.MapperWithBodyAndTypes(
+            "static partial B MapToB(this A source);",
+            "class A { public int Value { get; set; } }",
+            "class B { public int Value { get; set; } }"
+        );
+
+        return TestHelper.VerifyGenerator(source);
+    }
+
+    [Fact]
+    public Task ExtensionUpdateMethodShouldWork()
+    {
+        var source = TestSourceBuilder.MapperWithBodyAndTypes(
+            "static partial void MapToB(this A source, B target);",
+            "class A { public int Value { get; set; } }",
+            "class B { public int Value { get; set; } }"
+        );
+
+        return TestHelper.VerifyGenerator(source);
+    }
+}

test/Riok.Mapperly.Tests/_snapshots/ExtensionMethodTest.ExtensionMapMethodShouldWork#Mapper.g.verified.cs

@@ -0,0 +1,12 @@
+//HintName: Mapper.g.cs
+// <auto-generated />
+#nullable enable
+public partial class Mapper
+{
+    static partial global::B MapToB(this global::A source)
+    {
+        var target = new global::B();
+        target.Value = source.Value;
+        return target;
+    }
+}

test/Riok.Mapperly.Tests/_snapshots/ExtensionMethodTest.ExtensionUpdateMethodShouldWork#Mapper.g.verified.cs

@@ -0,0 +1,10 @@
+//HintName: Mapper.g.cs
+// <auto-generated />
+#nullable enable
+public partial class Mapper
+{
+    static partial void MapToB(this global::A source, global::B target)
+    {
+        target.Value = source.Value;
+    }
+}