Make mkFunCo take AnonArgFlags into accountwip/T21328

Previously, whenever `mkFunCo` would produce reflexive coercions, it would
use `mkVisFunTy` to produce the kind of the coercion. However, `mkFunCo` is
also used to produce coercions between types of the form `ty1 => ty2` in
certain places. This has the unfortunate side effect of causing the type of
the coercion to appear as `ty1 -> ty2` in certain error messages, as spotted
in #21328.

This patch address this by changing replacing the use of `mkVisFunTy` with
`mkFunctionType` in `mkFunCo`. `mkFunctionType` checks the kind of `ty1` and
makes the function arrow `=>` instead of `->` if `ty1` has kind `Constraint`,
so this should always produce the correct `AnonArgFlag`. As a result, this
patch fixes part (2) of #21328.

This is not the only possible way to fix #21328, as the discussion on that
issue lists some possible alternatives. Ultimately, it was concluded that the
alternatives would be difficult to maintain, and since we already use
`mkFunctionType` in `coercionLKind` and `coercionRKind`, using `mkFunctionType`
in `mkFunCo` is consistent with this choice. Moreover, using `mkFunctionType`
does not regress the performance of any test case we have in GHC's test suite.

0 files changed, 0 insertions, 0 deletions