Fix generic class pattern match widening solved type args to bound (#11526) by rchiodo · Pull Request #11527 · microsoft/pyright

rchiodo · 2026-06-25T16:38:22Z

Summary

Fixes #11526, a regression introduced in 1.1.411 by #11489 where a match against a generic class pattern loses an already-solved type argument and widens it to the type parameter's bound.

Problem

class Wrapper[A: str | list]:
    data: A

def failing_match_unpack(obj: Wrapper[list]) -> list:
    match obj:
        case Wrapper(data=data):
            return data  # error: "str | list[Unknown]" is not assignable to "list[Unknown]"

obj has a concrete type argument (list), but after the class pattern match data is narrowed to the full bound str | list instead of list.

Root cause

#11489 added specializeBoundedMatchTypeParams, which falls back to a bounded type parameter's bound when the parameter is left unsolved by the constraint solver. The guard that decides whether an argument was solved used:

if (specializedArg && !containsAnyOrUnknown(specializedArg, /* recurse */ true)) {
    return specializedArg;
}

With recurse: true, containsAnyOrUnknown descends into type arguments. So a concrete-but-implicitly-parameterized argument — bare list is list[Unknown], dict is dict[Unknown, Unknown], or explicit list[Any] — was treated as "unsolved" and replaced by the parameter's bound.

This matches every observation in the issue:

int (no params) and list[int] (no nested Unknown) → kept → works
bare list, dict, set, list[Any] → nested Unknown/Any found → widened to bound → fails

The genuine "unsolved" sentinel is a bare top-level Unknown (the case #11489 targeted, where the subject carries no type arguments). A solved list[Unknown] is a real list class and must be preserved.

Fix

Only treat a bare top-level Unknown as the unsolved sentinel:

if (specializedArg && !isUnknown(specializedArg)) {
    return specializedArg;
}

This matches the original intent ("keep an argument unless it is genuinely Unknown") while preserving concrete arguments.

Testing

Extended matchNamedTupleGeneric1.py with regression cases for Wrapper[list], Wrapper[list[Any]], Wrapper[list[int]], and MultiWrapper[dict]; the existing NamedTuple12 test continues to assert 0 errors.
All original Fix generic NamedTuple match narrowing for bounded type vars #11489 cases still pass (subject narrowed against object still falls back to the bound, in-scope TypeVar arguments preserved, generic-supertype case, etc.).
Full pyright-internal suite passes (the only failing suite is languageServer.test.ts, which requires the prebuilt test server bundle and is unrelated).

PR microsoft#11489 added a fallback that specializes unsolved bounded type parameters to their bound after a class pattern match. The guard that distinguishes a solved argument from an unsolved one used containsAnyOrUnknown(arg, recurse=true), which descends into type arguments. As a result a concrete-but-implicitly-parameterized argument (bare list = list[Unknown], dict, set, list[Any]) was misclassified as unsolved and widened to the parameter's bound, breaking previously valid code (issue microsoft#11526). Only treat a bare top-level Unknown - the constraint solver's unsolved sentinel - as needing the bound fallback, so concrete arguments are preserved. Fixes microsoft#11526 Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

github-actions · 2026-06-25T16:49:18Z

Diff from mypy_primer, showing the effect of this PR on open source code:

sympy (https://github.com/sympy/sympy)
-   .../projects/sympy/sympy/solvers/tests/test_solveset.py:315:25 - error: "Set" is not iterable
-     "__iter__" method not defined (reportGeneralTypeIssues)
-   .../projects/sympy/sympy/solvers/tests/test_solveset.py:315:25 - error: "ConditionSet" is not iterable
-     "__iter__" method not defined (reportGeneralTypeIssues)
-   .../projects/sympy/sympy/solvers/tests/test_solveset.py:2030:20 - error: "__getitem__" method not defined on type "Basic" (reportIndexIssue)
-   .../projects/sympy/sympy/solvers/tests/test_solveset.py:2042:20 - error: "__getitem__" method not defined on type "Basic" (reportIndexIssue)
-   .../projects/sympy/sympy/solvers/tests/test_solveset.py:2055:20 - error: "__getitem__" method not defined on type "Basic" (reportIndexIssue)
-   .../projects/sympy/sympy/solvers/tests/test_solveset.py:2076:16 - error: Argument of type "Unknown | FiniteSet | Set | Intersection | Union | Complement | Any | ConditionSet" cannot be assigned to parameter "obj" of type "Sized" in function "len"
-     Type "Unknown | FiniteSet | Set | Intersection | Union | Complement | Any | ConditionSet" is not assignable to type "Sized"
-       "Set" is incompatible with protocol "Sized"
-         "__len__" is not present (reportArgumentType)
-   .../projects/sympy/sympy/solvers/tests/test_solveset.py:2077:20 - error: "__getitem__" method not defined on type "Basic" (reportIndexIssue)
-   .../projects/sympy/sympy/solvers/tests/test_solveset.py:2270:16 - error: Argument of type "Unknown | FiniteSet | Set | Intersection | Union | Complement | Any | ConditionSet" cannot be assigned to parameter "obj" of type "Sized" in function "len"
-     Type "Unknown | FiniteSet | Set | Intersection | Union | Complement | Any | ConditionSet" is not assignable to type "Sized"
-       "Set" is incompatible with protocol "Sized"
-         "__len__" is not present (reportArgumentType)
-   .../projects/sympy/sympy/solvers/tests/test_solveset.py:2351:16 - error: Argument of type "Unknown | FiniteSet | Set | Intersection | Union | Complement | Any | ConditionSet" cannot be assigned to parameter "obj" of type "Sized" in function "len"
-     Type "Unknown | FiniteSet | Set | Intersection | Union | Complement | Any | ConditionSet" is not assignable to type "Sized"
-       "Set" is incompatible with protocol "Sized"
-         "__len__" is not present (reportArgumentType)
-   .../projects/sympy/sympy/solvers/tests/test_solveset.py:2364:16 - error: Argument of type "Unknown | FiniteSet | Set | Intersection | Union | Complement | Any | ConditionSet" cannot be assigned to parameter "obj" of type "Sized" in function "len"
-     Type "Unknown | FiniteSet | Set | Intersection | Union | Complement | Any | ConditionSet" is not assignable to type "Sized"
-       "Set" is incompatible with protocol "Sized"
-         "__len__" is not present (reportArgumentType)
-   .../projects/sympy/sympy/solvers/tests/test_solveset.py:2370:16 - error: Argument of type "Unknown | FiniteSet | Set | Intersection | Union | Complement | Any | ConditionSet" cannot be assigned to parameter "obj" of type "Sized" in function "len"
-     Type "Unknown | FiniteSet | Set | Intersection | Union | Complement | Any | ConditionSet" is not assignable to type "Sized"
-       "Set" is incompatible with protocol "Sized"
-         "__len__" is not present (reportArgumentType)
-   .../projects/sympy/sympy/solvers/tests/test_solveset.py:2538:16 - error: Argument of type "Unknown | FiniteSet | Set | Intersection | Union | Complement | Any | ConditionSet" cannot be assigned to parameter "obj" of type "Sized" in function "len"
-     Type "Unknown | FiniteSet | Set | Intersection | Union | Complement | Any | ConditionSet" is not assignable to type "Sized"
-       "Set" is incompatible with protocol "Sized"
-         "__len__" is not present (reportArgumentType)
-   .../projects/sympy/sympy/solvers/tests/test_solveset.py:2541:20 - error: Argument of type "Unknown | Basic | Any" cannot be assigned to parameter "obj" of type "Sized" in function "len"
-     Type "Unknown | Basic | Any" is not assignable to type "Sized"
-       "Basic" is incompatible with protocol "Sized"
-         "__len__" is not present (reportArgumentType)
-   .../projects/sympy/sympy/stats/crv_types.py:2544:9 - error: Method "_cdf" overrides class "SingleContinuousDistribution" in an incompatible manner
-     Return type mismatch: base method returns type "None", override returns type "ComplexInfinity | NaN | Rational | Unknown | Zero | Expr"
-       Type "ComplexInfinity | NaN | Rational | Unknown | Zero | Expr" is not assignable to type "None"
-         "Expr" is not assignable to "None" (reportIncompatibleMethodOverride)
-   .../projects/sympy/sympy/stats/crv_types.py:2723:9 - error: Method "_cdf" overrides class "SingleContinuousDistribution" in an incompatible manner
-     Return type mismatch: base method returns type "None", override returns type "Expr | NaN | ComplexInfinity | Rational | Unknown | One | NegativeOne | Zero | Integer"
-       Type "Expr | NaN | ComplexInfinity | Rational | Unknown | One | NegativeOne | Zero | Integer" is not assignable to type "None"
-         "Expr" is not assignable to "None" (reportIncompatibleMethodOverride)
-   .../projects/sympy/sympy/stats/drv.py:269:22 - error: Argument of type "Generator[tuple[Unknown, ...] | Unknown | Sum | Expr | ZeroMatrix | Add | Zero | NaN | Piecewise | Basic | int | None, None, None]" cannot be assigned to parameter "iterable" of type "Iterable[_SupportsSumNoDefaultT@sum]" in function "sum"
+   .../projects/sympy/sympy/stats/drv.py:269:22 - error: Argument of type "Generator[tuple[Unknown, ...] | Unknown | Sum | Expr | ZeroMatrix | Zero | NaN | Piecewise | Basic | int | None, None, None]" cannot be assigned to parameter "iterable" of type "Iterable[_SupportsSumNoDefaultT@sum]" in function "sum"
-     "Generator[tuple[Unknown, ...] | Unknown | Sum | Expr | ZeroMatrix | Add | Zero | NaN | Piecewise | Basic | int | None, None, None]" is not assignable to "Iterable[_SupportsSumNoDefaultT@sum]"
+     "Generator[tuple[Unknown, ...] | Unknown | Sum | Expr | ZeroMatrix | Zero | NaN | Piecewise | Basic | int | None, None, None]" is not assignable to "Iterable[_SupportsSumNoDefaultT@sum]"
-       Type parameter "_T_co@Iterable" is covariant, but "tuple[Unknown, ...] | Unknown | Sum | Expr | ZeroMatrix | Add | Zero | NaN | Piecewise | Basic | int | None" is not a subtype of "_SupportsSumNoDefaultT@sum"
+       Type parameter "_T_co@Iterable" is covariant, but "tuple[Unknown, ...] | Unknown | Sum | Expr | ZeroMatrix | Zero | NaN | Piecewise | Basic | int | None" is not a subtype of "_SupportsSumNoDefaultT@sum"
-         Type "tuple[Unknown, ...] | Unknown | Sum | Expr | ZeroMatrix | Add | Zero | NaN | Piecewise | Basic | int | None" is not assignable to type "_SupportsSumWithNoDefaultGiven"
+         Type "tuple[Unknown, ...] | Unknown | Sum | Expr | ZeroMatrix | Zero | NaN | Piecewise | Basic | int | None" is not assignable to type "_SupportsSumWithNoDefaultGiven"
-           Type "tuple[Unknown, ...] | Unknown | Sum | Expr | ZeroMatrix | Add | Zero | NaN | Piecewise | Basic | int | None" is not assignable to type "_SupportsSumWithNoDefaultGiven"
+           Type "tuple[Unknown, ...] | Unknown | Sum | Expr | ZeroMatrix | Zero | NaN | Piecewise | Basic | int | None" is not assignable to type "_SupportsSumWithNoDefaultGiven"
-   .../projects/sympy/sympy/stats/drv_types.py:293:16 - error: Operator "*" not supported for types "Expr" and "tuple[Unknown, ...] | Unknown | Sum | Expr | ZeroMatrix | Add | Zero | NaN | Piecewise | Basic"
+   .../projects/sympy/sympy/stats/drv_types.py:293:16 - error: Operator "*" not supported for types "Expr" and "tuple[Unknown, ...] | Unknown | Sum | Expr | ZeroMatrix | Zero | NaN | Piecewise | Basic"
-   .../projects/sympy/sympy/stats/frv_types.py:139:17 - error: Argument of type "NaN | ComplexInfinity | Rational | Unknown | Expr" cannot be assigned to parameter "value" of type "int" in function "__setitem__"
-     Type "NaN | ComplexInfinity | Rational | Unknown | Expr" is not assignable to type "int"
-       "Expr" is not assignable to "int" (reportArgumentType)
-   .../projects/sympy/sympy/stats/random_matrix_models.py:111:36 - error: Operator "*" not supported for types "Expr" and "tuple[Unknown, ...] | Unknown | Product | Basic | BooleanFalse | BooleanTrue | ComplexInfinity | Equality | Expr | Float | Infinity | Integer | NaN | NegativeInfinity | NegativeOne | Number | One | Rational | Zero"
+   .../projects/sympy/sympy/stats/random_matrix_models.py:111:36 - error: Operator "*" not supported for types "Expr" and "tuple[Unknown, ...] | Unknown | Product | Basic | BooleanFalse | BooleanTrue | Equality | Expr | NaN | One | Zero"
-   .../projects/sympy/sympy/stats/rv.py:473:25 - error: Argument of type "Expr | Lambda | Zero | One | Integral | Unknown | Probability | tuple[Unknown, ...] | Sum | ZeroMatrix | Add | NaN | Piecewise | Basic | NegativeOne | Integer | ComplexInfinity | Rational | Infinity | NegativeInfinity | Float | Number | int" cannot be assigned to parameter "args" of type "Expr | complex" in function "__new__"
-     Type "Expr | Lambda | Zero | One | Integral | Unknown | Probability | tuple[Unknown, ...] | Sum | ZeroMatrix | Add | NaN | Piecewise | Basic | NegativeOne | Integer | ComplexInfinity | Rational | Infinity | NegativeInfinity | Float | Number | int" is not assignable to type "Expr | complex"
+   .../projects/sympy/sympy/stats/rv.py:473:25 - error: Argument of type "Expr | Lambda | Zero | One | Integral | Unknown | NegativeOne | Integer | NaN | ComplexInfinity | Rational | Infinity | Number | NegativeInfinity | Probability | tuple[Unknown, ...] | Sum | ZeroMatrix | Piecewise | Basic | int" cannot be assigned to parameter "args" of type "Expr | complex" in function "__new__"

... (truncated 1093 lines) ...

rchiodo enabled auto-merge (squash) June 25, 2026 16:49

bschnurr approved these changes Jun 25, 2026

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rchiodo merged commit e1f6805 into microsoft:main Jun 25, 2026
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Fix generic class pattern match widening solved type args to bound (#11526)#11527

Fix generic class pattern match widening solved type args to bound (#11526)#11527
rchiodo merged 1 commit into
microsoft:mainfrom
rchiodo:rchiodo-fix-match-bounded-typevar-narrowing

rchiodo commented Jun 25, 2026

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rchiodo commented Jun 25, 2026

Summary

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Fix

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