Holes

Wildcards could be allowed in the grammar of expressions. Such a construction would always produce non-fatal error, but after the effect inference the compiler could print some useful information about the context, like the expected type and effect, and types of some local variables.

Start by locating the grammar for expressions and the compiler's effect-inference entry points, then review how non-fatal errors are represented. Done would require agreed semantics for wildcard expressions and a defined way to report inferred context, expected types, effects, and local-variable types.

Written by the indexing model from the issue text.