feat: enhance type checking by excluding nullish types in assignability checks

Author: nicosammito

src/util/nodes.util.ts

@@ -79,8 +79,16 @@ const createNodeFunctionIfCompatible = (
     // Simplify the return type by resolving type parameters
     const simplifiedReturnType = resolveReturnType(checker, returnType);
 
-    // Only proceed if the return type is assignable to the target parameter type
-    if (!checker.isTypeAssignableTo(simplifiedReturnType, paramType)) {
+    // Only proceed if the return type is assignable to the target parameter type.
+    // The nullish part of the return type is ignored: a function returning
+    // `string | null` is still a valid suggestion for a plain `string` parameter.
+    // A purely nullish return type strips down to `never` (assignable to anything),
+    // so it must be excluded explicitly.
+    const nonNullableReturnType = checker.getNonNullableType(simplifiedReturnType);
+    if (
+        (nonNullableReturnType.flags & ts.TypeFlags.Never) !== 0 ||
+        !checker.isTypeAssignableTo(nonNullableReturnType, paramType)
+    ) {
         return null;
     }
 

src/util/references.util.ts

@@ -257,8 +257,17 @@ const extractObjectProperties = (
 ): Array<{ path: ReferencePath[]; type: ts.Type }> => {
   const results: Array<{ path: ReferencePath[]; type: ts.Type }> = [];
 
-  // Check if the current type matches the expected type
-  if (checker.isTypeAssignableTo(type, expectedType)) {
+  // Check if the current type matches the expected type. The nullish part of a
+  // candidate is ignored: a reference typed `string | null` (or an optional
+  // property `text?: string | null`) is still a valid suggestion for a plain
+  // `string` parameter — strict assignability would reject it under strictNullChecks.
+  // A purely nullish candidate strips down to `never` (assignable to anything),
+  // so it must be excluded explicitly.
+  const nonNullableType = checker.getNonNullableType(type);
+  if (
+    (nonNullableType.flags & ts.TypeFlags.Never) === 0 &&
+    checker.isTypeAssignableTo(nonNullableType, expectedType)
+  ) {
     results.push({ path: currentPath, type });
   }