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Fixed bug that results in an "incompatible method override" false neg… #9707

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merged 1 commit into from
Jan 14, 2025
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Fixed bug that results in an "incompatible method override" false neg… #9707

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3 changes: 2 additions & 1 deletion packages/pyright-internal/src/analyzer/checker.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -6619,7 +6619,7 @@ export class Checker extends ParseTreeWalker {
selfSpecializeClass(childClassType, { useBoundTypeVars: true })
);

const baseType = partiallySpecializeType(
let baseType = partiallySpecializeType(
this._evaluator.getEffectiveTypeOfSymbol(baseClassAndSymbol.symbol),
baseClass,
this._evaluator.getTypeClassType(),
Expand All @@ -6635,6 +6635,7 @@ export class Checker extends ParseTreeWalker {

if (childClassType.shared.typeVarScopeId) {
overrideType = makeTypeVarsBound(overrideType, [childClassType.shared.typeVarScopeId]);
baseType = makeTypeVarsBound(baseType, [childClassType.shared.typeVarScopeId]);
}

if (isFunction(baseType) || isOverloaded(baseType)) {
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10 changes: 10 additions & 0 deletions packages/pyright-internal/src/tests/samples/methodOverride1.py
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Expand Up @@ -521,10 +521,20 @@ def method1(self, x: T) -> T:


class Derived7_1(Base7[T]):
# This should generate an error.
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Could you please explain why this should generate an error?

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Consider the following:

d = Derived7_1[int]()
d.method1("")

def func(b: Base7[int]):
    b.method1(1)

func(d)

For an instance of Derived7_1[int], the specialized type of the base class method is (int) -> int, and the specialized type of the derived class method is (str) -> str. It's a Liskov violation.

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@LeeeeT LeeeeT Jan 15, 2025 •
edited
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Derived7_1.method1 extends Base7.method1 by making it polymorphic. (S@method1) -> S@method1 is assignable to (T) -> T for any concrete T that comes from the generic specialization, isn't it? I don't know what the intended behavior is, but it feels inconsistent with what was implemented in #9417. That's why I decided to point that out.

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This is different from the case you reported in #9417 because that didn't involve a class-scoped type variable in the base class. In this case, the override is unsound as demonstrated in the sample above.

def method1(self, x: S) -> S:
return x


class Derived7_2(Base7[int]):
def method1(self, x: U) -> U:
return x


class Base8[T]:
def method1(self, x: T) -> T: ...


class Derived8[T](Base8[T]):
# This should generate an error.
def method1[U: str](self, x: U) -> U: ...
2 changes: 1 addition & 1 deletion packages/pyright-internal/src/tests/typeEvaluator3.test.ts
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Expand Up @@ -946,7 +946,7 @@ test('MethodOverride1', () => {

configOptions.diagnosticRuleSet.reportIncompatibleMethodOverride = 'error';
analysisResults = TestUtils.typeAnalyzeSampleFiles(['methodOverride1.py'], configOptions);
TestUtils.validateResults(analysisResults, 40);
TestUtils.validateResults(analysisResults, 42);
});

test('MethodOverride2', () => {
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