improvement: ignore empty variants for implicit conversions

src/Dunet.Generator/UnionGeneration/UnionDeclaration.cs

@@ -18,33 +18,44 @@ ImmutableEquatableArray<Property> Properties
     // It also doesn't make sense to generate Match extensions if there are no variants to match against.
     public bool SupportsExtensionMethods() => Namespace is not null && Variants.Count > 0;
 
-    public bool SupportsImplicitConversions()
+    public List<VariantDeclaration> VariantsWithImplicitConversionSupport()
     {
-        var allVariantsHaveSingleParameter = () =>
-            Variants.All(static variant => variant.Parameters.Count is 1);
+        var hasRequiredProperties = Properties.Any(static property => property.IsRequired);
 
-        var noVariantHasInterfaceParameter = () =>
-            Variants
-                .SelectMany(static variant => variant.Parameters)
-                .All(static parameter => !parameter.Type.IsInterface);
+        // We cannot generate implicit conversions for unions with required properties because
+        // we cannot initialize the required value as part of performing the conversion.
+        if (hasRequiredProperties)
+        {
+            return [];
+        }
 
-        var allVariantsParameterTypesAreDifferent = () =>
+        static bool eachVariantHasUniqueParameterType(IEnumerable<VariantDeclaration> variants)
         {
-            var allParameterTypes = Variants
+            var allParameterTypes = variants
+                // Ignore variants with no parameters since they don't impact uniqueness.
+                .Where(static variant => variant.Parameters.Count > 0)
                 .SelectMany(static variant => variant.Parameters)
-                .Select(static parameter => parameter.Type.Identifier);
-            var numAllParameterTypes = allParameterTypes.Count();
+                .Select(static parameter => parameter.Type.Identifier)
+                .ToList();
+            var numAllParameterTypes = allParameterTypes.Count;
             var numUniqueParameterTypes = allParameterTypes.Distinct().Count();
             return numAllParameterTypes == numUniqueParameterTypes;
-        };
+        }
+
+        if (!eachVariantHasUniqueParameterType(Variants))
+        {
+            return [];
+        }
+
+        // Isolate the variants that have a single parameter because only those can have implicit conversions.
+        bool hasSingleParameter(VariantDeclaration variant) => variant.Parameters.Count is 1;
 
-        var unionHasNoRequiredProperties = () =>
-            !Properties.Any(static property => property.IsRequired);
+        static bool hasInterfaceParameter(VariantDeclaration variant) =>
+            variant.Parameters.Any(static parameter => parameter.Type.IsInterface);
 
-        return allVariantsHaveSingleParameter()
-            && noVariantHasInterfaceParameter()
-            && allVariantsParameterTypesAreDifferent()
-            && unionHasNoRequiredProperties();
+        return Variants
+            .Where(variant => hasSingleParameter(variant) && !hasInterfaceParameter(variant))
+            .ToList();
     }
 }
 

src/Dunet.Generator/UnionGeneration/UnionSourceBuilder.cs

@@ -40,18 +40,15 @@ public static string Build(UnionDeclaration union)
         builder.AppendAbstractSpecificMatchMethods(union);
         builder.AppendAbstractUnwrapMethods(union);
 
-        if (union.SupportsImplicitConversions())
+        foreach (var variant in union.VariantsWithImplicitConversionSupport())
         {
-            foreach (var variant in union.Variants)
-            {
-                builder.Append($"    public static implicit operator {union.Name}");
-                builder.AppendTypeParams(union.TypeParameters);
-                builder.AppendLine(
-                    $"({variant.Parameters[0].Type.Identifier} value) => new {variant.Identifier}(value);"
-                );
-            }
-            builder.AppendLine();
+            builder.Append($"    public static implicit operator {union.Name}");
+            builder.AppendTypeParams(union.TypeParameters);
+            builder.AppendLine(
+                $"({variant.Parameters[0].Type.Identifier} value) => new {variant.Identifier}(value);"
+            );
         }
+        builder.AppendLine();
 
         foreach (var variant in union.Variants)
         {

test/UnionGeneration/ImplicitConversionTests.cs

@@ -142,4 +142,32 @@ partial record Failure(IEnumerable<string> Errors);
         result.CompilationErrors.Should().BeEmpty();
         result.GenerationDiagnostics.Should().BeEmpty();
     }
+
+    [Fact]
+    public void IgnoresEmptyMembersWhenGeneratingImplicitConversions()
+    {
+        var programCs = """
+            using Dunet;
+            using System;
+            using static Option<int>;
+
+            Option<int> success = 42;
+            Option<int> none = new None();
+
+            [Union]
+            partial record Option<T>
+            {
+                partial record Some(T Value);
+                partial record None();
+            }
+            """;
+
+        // Act.
+        using var scope = new AssertionScope();
+        var result = Compiler.Compile(programCs);
+
+        // Assert.
+        result.CompilationErrors.Should().BeEmpty();
+        result.GenerationDiagnostics.Should().BeEmpty();
+    }
 }