[commit: ghc] wip/T16191: Control validity-checking of type synonym applications more carefully (9dc56b6)

[git at git.haskell.org]

Repository : ssh://git@git.haskell.org/ghc

On branch  : wip/T16191
Link       : http://ghc.haskell.org/trac/ghc/changeset/9dc56b61ae10cb26ec8b2bc132ae6c9b620707a8/ghc

>---------------------------------------------------------------

commit 9dc56b61ae10cb26ec8b2bc132ae6c9b620707a8
Author: Ryan Scott <ryan.gl.scott at gmail.com>
Date:   Thu Jan 10 16:55:51 2019 -0500

    Control validity-checking of type synonym applications more carefully
    
    Trac #16059 shows that when validity checking applications of type
    synonyms, GHC sometimes wasn't checking the expanded type enough.
    We must be careful, however, since checking both the expanded type as
    well as the arguments to the type synonym can lead to exponential
    blowup (see https://ghc.haskell.org/trac/ghc/ticket/16059#comment:4).
    Nor can we omit checking either the expanded type or the argument for
    correctness reasons.
    
    The solution here is to introduce a new `ExpandMode` data type that
    is plumbed through all of the type-validity-checking functions in
    `TcValidity`. `ExpandMode` dictates whether we only check the
    expanded type (`Expand`), only check the arguments (`NoExpand), or
    both (`Both`). Importantly, if we check `Both` in the function for
    validity checking type synonym applications, then we switch to
    `NoExpand` when checking the arguments so as to avoid exponential
    blowup. See `Note [Correctness and performance of type synonym validity
    checking]` for the full story.


>---------------------------------------------------------------

9dc56b61ae10cb26ec8b2bc132ae6c9b620707a8
 compiler/typecheck/TcValidity.hs                   | 241 ++++++++++++++++-----
 testsuite/tests/safeHaskell/unsafeLibs/Dep05.hs    |   1 +
 .../tests/safeHaskell/unsafeLibs/Dep05.stderr      |   2 +-
 testsuite/tests/typecheck/should_fail/T16059a.hs   |  12 +
 .../tests/typecheck/should_fail/T16059a.stderr     |   9 +
 testsuite/tests/typecheck/should_fail/T16059b.hs   |   6 +
 testsuite/tests/typecheck/should_fail/T16059c.hs   |   6 +
 .../tests/typecheck/should_fail/T16059c.stderr     |   6 +
 testsuite/tests/typecheck/should_fail/T16059d.hs   |   5 +
 .../tests/typecheck/should_fail/T16059d.stderr     |   6 +
 testsuite/tests/typecheck/should_fail/T16059e.hs   |  15 ++
 .../tests/typecheck/should_fail/T16059e.stderr     |   7 +
 .../tests/typecheck/should_fail/T9858a.stderr      |  18 +-
 testsuite/tests/typecheck/should_fail/all.T        |   7 +
 14 files changed, 271 insertions(+), 70 deletions(-)

Diff suppressed because of size. To see it, use:

    git diff-tree --root --patch-with-stat --no-color --find-copies-harder --ignore-space-at-eol --cc 9dc56b61ae10cb26ec8b2bc132ae6c9b620707a8