Do not abort constructing TypeAlias if only type parameters hold us back. #20162
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sterliakov
commented
Nov 1, 2025
| is_error = False | ||
| is_invalid = False | ||
| for (i, arg), tvar in zip(enumerate(args), type_vars): | ||
| for arg, tvar in zip(args, type_vars): |
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This change does not belong here, but I don't think it warrants a separate PR - i was just unused, I noticed it when tracing a PlaceholderType
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According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅ |
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Fixes #20135.
I am not fully certain that this is the right way. If the type alias can be reasonably constructed and only type parameters prevent that, we can create an "almost" equivalent alias with all placeholder-bearing typevars replaced with their "simple" equivalents with default values/bound/default values. This would cause current iteration to not emit some parameterization errors later, but, since we already defer the alias as a whole, we will recheck all of them again.
This obviously adds some pointless work (we check parameterization that will not be used later), but probably is not that big of a deal, recursive aliases are relatively rare in wild. If this turns out to be a bottleneck, we can add a
parameters_readyflag to aliases and skip the heavy parts if it is False, but I think it isn't necessary now.