Sema: Restore old behavior of generic parameters with associated type inference #71548
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… inference This effectively reverts d0bd026, so we now look for abstract type witnesses before generic parameters. In particular, this means we again prefer the default type witness over a generic parameter if nothing else forces it to be a generic parameter: protocol P { associatedtype A = Int } struct S<T>: P {} // S.T is always Int Fixing this properly requires modeling generic parameter bindings as disjunctions, which is a more disruptive change than I want to take for now. Fixes rdar://problem/122587432.
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| // RUN: %target-typecheck-verify-swift -disable-experimental-associated-type-inference | ||
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| protocol P { | ||
| associatedtype A = Int | ||
| } | ||
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| struct S<A>: P {} | ||
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| let x: String.Type = S<String>.A.self | ||
| // This is unfortunate but it is the behavior of Swift 5.10. | ||
| let x: Int.Type = S<String>.A.self |
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Would looking for generic parameters before defaults but after same-type requirements not fix this at least?
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Maybe, but I would rather fix this in one shot by building a disjunction containing the default associated type and generic parameter, if any, in inferTypeWitnessesViaAssociatedType(), and having the solver walk through the solution space. This first requires better ranking rules and a smarter solver that won't go exponential as easily.
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This effectively reverts d0bd026, so
we now look for abstract type witnesses before generic parameters.
In particular, this means we again prefer the default type witness
over a generic parameter if nothing else forces it to be a generic
parameter:
Fixing this properly requires modeling generic parameter bindings as
disjunctions, which is a more disruptive change than I want to take
for now.
Fixes rdar://problem/122587432.