Fix custom builders with import prefixes.

built_value_generator/lib/src/value_source_class.dart

@@ -707,7 +707,7 @@ abstract class ValueSourceClass
     if (hasBuilder) {
       for (var field in fields) {
         final type = field.typeInCompilationUnit(compilationUnit);
-        final typeInBuilder = field.typeInBuilder(compilationUnit);
+        final typeInBuilder = field.builderElementTypeWithPrefix;
         final name = field.name;
 
         if (field.isNestedBuilder) {

built_value_generator/lib/src/value_source_field.dart

@@ -124,15 +124,44 @@ abstract class ValueSourceField
     // Try to get a resolved type first, it's faster.
     var result = builderElement.getter?.returnType?.displayName;
     if (result != null && result != 'dynamic') return result;
-    // Go via AST to allow use of unresolvable types not yet generated.
-    return parsedLibrary
+    // Go via AST to allow use of unresolvable types not yet generated;
+    // this includes generated Builder types.
+    result = parsedLibrary
             .getElementDeclaration(builderElement)
             ?.node
             ?.parent
             ?.childEntities
             ?.first
             .toString() ??
         'dynamic';
+    // If we went via the AST there may be an import prefix, but we don't
+    // want one here. Strip it off.
+    if (result.contains('.')) {
+      result = result.substring(result.indexOf('.') + 1);
+    }
+    return result;
+  }
+
+  /// The [builderElementType] plus any import prefix.
+  @memoized
+  String get builderElementTypeWithPrefix {
+    // If it's a real field, it's a [VariableDeclaration] which is guaranteed
+    // to have parent node [VariableDeclarationList] giving the type.
+    var fieldDeclaration = parsedLibrary.getElementDeclaration(builderElement);
+    if (fieldDeclaration != null) {
+      return (((fieldDeclaration.node as VariableDeclaration).parent)
+                  as VariableDeclarationList)
+              ?.type
+              ?.toSource() ??
+          'dynamic';
+    } else {
+      // Otherwise it's an explicit getter/setter pair; get the type from the getter.
+      return (parsedLibrary.getElementDeclaration(builderElement.getter).node
+                  as MethodDeclaration)
+              ?.returnType
+              ?.toSource() ??
+          'dynamic';
+    }
   }
 
   /// Gets the type name for the builder. Specify the compilation unit to

end_to_end_test/lib/imported_values.dart

@@ -22,3 +22,54 @@ abstract class ImportedValue
       _$ImportedValue;
   ImportedValue._();
 }
+
+// With a custom builder.
+abstract class ImportedCustomValue
+    implements Built<ImportedCustomValue, ImportedCustomValueBuilder> {
+  static Serializer<ImportedCustomValue> get serializer =>
+      _$importedCustomValueSerializer;
+
+  prefix.SimpleValue get simpleValue;
+  BuiltList<prefix.SimpleValue> get simpleValues;
+
+  factory ImportedCustomValue(
+          [void Function(ImportedCustomValueBuilder) updates]) =
+      _$ImportedCustomValue;
+  ImportedCustomValue._();
+}
+
+abstract class ImportedCustomValueBuilder
+    implements Builder<ImportedCustomValue, ImportedCustomValueBuilder> {
+  prefix.SimpleValue simpleValue;
+  BuiltList<prefix.SimpleValue> simpleValues;
+
+  ImportedCustomValueBuilder._();
+  factory ImportedCustomValueBuilder() = _$ImportedCustomValueBuilder;
+}
+
+// With a custom builder using nested buliders.
+abstract class ImportedCustomNestedValue
+    implements
+        Built<ImportedCustomNestedValue, ImportedCustomNestedValueBuilder> {
+  static Serializer<ImportedCustomNestedValue> get serializer =>
+      _$importedCustomNestedValueSerializer;
+
+  prefix.SimpleValue get simpleValue;
+  BuiltList<prefix.SimpleValue> get simpleValues;
+
+  factory ImportedCustomNestedValue(
+          [void Function(ImportedCustomNestedValueBuilder) updates]) =
+      _$ImportedCustomNestedValue;
+  ImportedCustomNestedValue._();
+}
+
+abstract class ImportedCustomNestedValueBuilder
+    implements
+        Builder<ImportedCustomNestedValue, ImportedCustomNestedValueBuilder> {
+  prefix.SimpleValueBuilder simpleValue;
+  ListBuilder<prefix.SimpleValue> simpleValues;
+
+  ImportedCustomNestedValueBuilder._();
+  factory ImportedCustomNestedValueBuilder() =
+      _$ImportedCustomNestedValueBuilder;
+}