Refine implicit search fallbacks for better ClassTag handling #23532
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It's not necessary to instantiate all type variables, deeply, since we are only interested in the outermost shape. Also, that way we do not instantiate to Nothing, which is something difficult to recover from.
We used to look at deep expected type when the result of an implicit was ambiguous. This could add more constraints which could resolve the ambiguity. We now do the same also if the search type has uninstantiated type variables. In that case, consulting more context might further constrain type variables, which might in turn enlarge the implicit scope so that a solution can be found. Fixes scala#23526
odersky
changed the title
lrytz
approved these changes
Jul 17, 2025
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It's not necessary to instantiate all type variables, deeply, since we are only interested in the outermost shape. Also, that way we do not instantiate to Nothing, which is something difficult to recover from. This step is necessary to enable (2)
We used to look at the deep expected type when the result of an implicit was ambiguous.
This could add more constraints which could resolve the ambiguity.
We now do the same also if the search type has uninstantiated type variables. In that
case, consulting more context might further constrain type variables, which might in
turn enlarge the implicit scope so that a solution can be found.
Fixes #23526