fix: use maxType when building expression in expression tree elaborator #4215
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After the expression tree elaborator inserts coercions, it builds the expression itself. It was not storing the computed maxType in the return types of operators. This led to unexpected errors in examples such as ```lean example (a : Nat) (b : Int) : a = id (a * b^2) := sorry ``` where it would say it could not synthesize an `HMul Int Int Nat` instance. The issue in this case is that `HPow` uses default instances, so while the expression tree elaborator decides that `a * b^2` should be referring to an `Int`, the actual elaborated type is temporarily a metavariable, so when the binrel elaborator is looking at both sides of the equality, it decides that `Nat` will work and coercions don't need to be inserted. The fix is to have the expression tree elaborator elaborate all the operators using the computed `maxType` as the expected type.
kmill
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May 18, 2024
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The expression tree elaborator computes a "maxType" that every leaf term can be coerced to, but the elaborator was not ensuring that the entire expression tree would have maxType as its type. This led to unexpected errors in examples such as
where it would say it could not synthesize an
HMul Int Int Natinstance (theNatwould propagate from theaon the LHS of the equality). The issue in this case is thatHPowuses default instances, so while the expression tree elaborator decides thata * b^2should be referring to anInt, the actual elaborated type is temporarily a metavariable. Then, when the binrel elaborator is looking at both sides of the equality, it decides thatNatwill work and coercions don't need to be inserted.The fix is to unify the type of the resulting elaborated expression with the computed maxType. One wrinkle is that
hasUncomparablebeing false is a valid test only if there are no leaf terms with unknown types (if they become known, it could changehasUncomparableto true), so this unification is only performed if the leaf terms all have known types.Fixes issue described by Floris van Doorn on Zulip.