fix: use correct nullable types when referencing named mappings (#1237)

Author: latonz

src/Riok.Mapperly/Descriptors/MappingBuilderContext.cs

@@ -175,7 +175,16 @@ bool ignoreDerivedTypes
         Location? diagnosticLocation = null
     )
     {
-        return FindMapping(key) ?? FindOrBuildMapping(key.NonNullable(), options, diagnosticLocation);
+        if (FindMapping(key) is { } mapping)
+            return mapping;
+
+        // if a user mapping is referenced
+        // build it with the exact types
+        // as it may expect a nullable source type
+        if (key.Configuration.UseNamedMapping != null)
+            return BuildMapping(key, options, diagnosticLocation);
+
+        return FindOrBuildMapping(key.NonNullable(), options, diagnosticLocation);
     }
 
     /// <summary>

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

@@ -347,4 +347,34 @@ class B
         );
         return TestHelper.VerifyGenerator(source);
     }
+
+    [Fact]
+    public Task ShouldPassNullValueToNullableUserMappingMethod()
+    {
+        var source = TestSourceBuilder.MapperWithBodyAndTypes(
+            """
+            [MapProperty(nameof(A.Value), nameof(B.Value), Use = nameof(MapString)]
+            public partial B Map(A source);
+
+            [UserMapping(Default = false)]
+            private string MapString(string? source)
+                => source ?? "(null)";
+            """,
+            """
+            class A
+            {
+                public string? Value { get; }
+                public string? Value2 { get; }
+            }
+            """,
+            """
+            class B
+            {
+                public string Value { set; }
+                public string Value2 { set; }
+            }
+            """
+        );
+        return TestHelper.VerifyGenerator(source);
+    }
 }

test/Riok.Mapperly.Tests/_snapshots/ObjectPropertyUseNamedMappingTest.ShouldPassNullValueToNullableUserMappingMethod#Mapper.g.verified.cs

@@ -0,0 +1,17 @@
+//HintName: Mapper.g.cs
+// <auto-generated />
+#nullable enable
+public partial class Mapper
+{
+    [global::System.CodeDom.Compiler.GeneratedCode("Riok.Mapperly", "0.0.1.0")]
+    public partial global::B Map(global::A source)
+    {
+        var target = new global::B();
+        if (source.Value2 != null)
+        {
+            target.Value2 = source.Value2;
+        }
+        target.Value = MapString(source.Value);
+        return target;
+    }
+}