inference: fix precision regression

[vtjnash]

While in 1d08d70 we wanted to avoid combining two types, we did not intend to prevent a type from being just a pass-through value.

[KristofferC]

Backporting this doesn't pass the added test https://build.julialang.org/#/builders/71/builds/3148/steps/5/logs/stdio. Should we just not backport it then?

[martinholters]

Should rather try to figure out how to make them pass, as they used to on 1.4, so this fixes a real regression.

[KristofferC]

Help appreciated :P

[martinholters]

On 1.5, we're getting a NOT_FOUND there. So using

    (typea === Union{} || typea === NOT_FOUND) && return typeb
    (typeb === Union{} || typeb === NOT_FOUND) && return typea

makes the test pass. But I'm not sure that is valid. I'm also not sure the NOT_FOUND is even supposed to propagate up to here. I have no idea what it is supposed to signify here and where it comes from.

[martinholters]

Prior to 1d08d70 we had tmerge(NOT_FOUND, T) == tmerge(T, NOT_FOUND) == T for any T (including Union{}). So that would mean the corresponding change to backport would be

typea === NOT_FOUND && return typeb
typeb === NOT_FOUND && return typea
typea === Union{} && return typeb
typeb === Union{} && return typea

On master, we seem to no longer see a NOT_FOUND in tmerge. So alternatively, we could consider backporting whichever change is responsible for that.

[martinholters]

Aha, alternatively I guess we could backport this part of dcc0696:

--- a/base/compiler/inferencestate.jl
+++ b/base/compiler/inferencestate.jl
@@ -210,7 +210,10 @@ update_valid_age!(edge::InferenceState, sv::InferenceState) = update_valid_age!(
 function record_ssa_assign(ssa_id::Int, @nospecialize(new), frame::InferenceState)
     old = frame.src.ssavaluetypes[ssa_id]
     if old === NOT_FOUND || !(new ⊑ old)
-        frame.src.ssavaluetypes[ssa_id] = tmerge(old, new)
+        # typically, we expect that old ⊑ new (that output information only
+        # gets less precise with worse input information), but to actually
+        # guarantee convergence we need to use tmerge here to ensure that is true
+        frame.src.ssavaluetypes[ssa_id] = old === NOT_FOUND ? new : tmerge(old, new)
         W = frame.ip
         s = frame.stmt_types
         for r in frame.ssavalue_uses[ssa_id]

[KristofferC]

Okay, I backported that part. Let's see how the tests do after that.