Fix varargs overload resolution with wildcard types #24669
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tanishiking
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Dec 4, 2025
Fixes scala#24072 When comparing overloaded methods where one is non-varargs with wildcard types (e.g., `Class[? <: T]`) and another is varargs, the non-varargs method should be preferred. Previously, the compiler failed to distinguish these methods , and results in an ambiguity error. ```scala def blub[T](a: Class[? <: T]): Unit // m1 def blub[T](a: Class[T], ints: Int*): Unit // m2 blub(classOf[Object]) // m1 should be picked, but fails to resolve ```` The problem is `compare(m1, m2)` returned 0 because: - (1). `m2` (varargs) is correctly considered "not as good" as `m1`. - (2). `m1` (non-varargs) was also considered "not as good" as `m2`. (but `m1` should be as good as `m2`! because `Class[Concrete]` can be applied to both m1 and m2). The (2) occurred because `Class[? <: T]` is not a subtype of `Class[T]` (due to invariance). Consequently, `isApplicableMethodRef(m2, Class[? <: T])` returned `false` because `isCompatible(Class[? <: T], Class[T])` returned `false` during the applicability check against the method. This commit adds special handling in `TestApplication.argOK` to check if wildcard upper bounds are compatible with their formal types during overload resolution, in addition to `isCompatible`.
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Fixes #24072
When comparing overloaded methods where one is non-varargs with wildcard types (e.g.,
Class[? <: T]) and another is varargs, the non-varargs method should be preferred.Previously, the compiler failed to distinguish these methods , and results in an ambiguity error.
The problem is
compare(m1, m2)returned 0 because:m2(varargs) is correctly considered "not as good" asm1.m1(non-varargs) was also considered "not as good" asm2. (butm1should be as good asm2! becauseClass[Concrete]can be applied to both m1 and m2).The (2) occurred because
Class[? <: T]is not a subtype ofClass[T](due to invariance). Consequently,isApplicableMethodRef(m2, Class[? <: T])returnedfalsebecauseisCompatible(Class[? <: T], Class[T])returnedfalseduring the applicability check against the method.This commit adds special handling in
TestApplication.argOKto check if wildcard upper bounds are compatible with their formal types during overload resolution, in addition toisCompatible.