• eir@cis.upenn.edu's avatar
    Do proper depth checking in the flattener to avoid looping. · c1edbdfd
    eir@cis.upenn.edu authored
    This implements (roughly) the plan put forward in comment:14:ticket:7788,
    fixing #7788, #8550, #9554, #10139, and addressing concerns raised in #10079.
    There are some regressions w.r.t. GHC 7.8, but only with pathological type
    families (like F a = F a). This also (hopefully -- don't have a test case)
    fixes #10158. Unsolved problems include #10184 and #10185, which are both
    known deficiencies of the approach used here.
    As part of this change, the plumbing around detecting infinite loops has
    changed. Instead of -fcontext-stack and -ftype-function-depth, we now have
    one combined -freduction-depth parameter. Setting it to 0 disbales the
    check, which is now the recommended way to get (terminating) code to
    typecheck in releases. (The number of reduction steps may well change between
    minor GHC releases!)
    This commit also introduces a new IntWithInf type in BasicTypes
    that represents an integer+infinity. This type is used in a few
    places throughout the code.
    Tests in
      typecheck/should_compile/T10184  (expected broken)
      typecheck/should_compile/T10185  (expected broken)
    This commit also changes performance testsuite numbers, for the better.
TcFlatten.hs 62.7 KB