Narrow union disambiguation to str, preserving the structural-ambiguity guard

The general exact-type tie-break removed the guard that makes structurally-
identical union variants (e.g. Union[A, B] dataclasses, Union[int, UnsignedInt])
raise on to_literal -- they would then be indistinguishable on read-back. Keep
that guard; only a bare string prefers the str variant over an enum-by-value.

Signed-off-by: 1fanwang <1fannnw@gmail.com>

Author: 1fanwang

flytekit/core/type_engine.py

@@ -2097,9 +2097,9 @@ async def async_to_literal(
             raise TypeTransformerFailedError(f"Cannot convert from {python_val} to {python_type}")
 
         if len(matches) > 1:
-            # More than one variant matched. Prefer the one whose type is exactly the value's type -- e.g. for
-            # Union[str, Color] and "red", both str and Color (which accepts a matching string) match, but str wins.
-            exact = [m for m in matches if type(python_val) is m[0]]
+            # A bare string matches both `str` and an enum whose values include it -- prefer `str` (the exact type).
+            # Every other multi-variant match stays ambiguous: structurally-identical variants are a read-back hazard.
+            exact = [m for m in matches if m[0] is str] if type(python_val) is str else []
             if len(exact) != 1:
                 raise TypeError(
                     "Ambiguous choice of variant for union type.\n"

tests/flytekit/unit/core/test_type_engine.py

@@ -2047,9 +2047,8 @@ def to_python_value(self, ctx: FlyteContext, lv: Literal, expected_python_type:
     assert union_type_tags_unique(lt)
 
     ctx = FlyteContextManager.current_context()
-    # int and UnsignedInt both accept 3, but int is the value's exact type, so it wins without ambiguity.
-    lv = TypeEngine.to_literal(ctx, 3, pt, lt)
-    assert lv.scalar.union.stored_type.structure.tag == "int"
+    with pytest.raises(TypeError, match="Ambiguous choice of variant for union type"):
+        TypeEngine.to_literal(ctx, 3, pt, lt)
 
     del TypeEngine._REGISTRY[UnsignedInt]