Inference regression with === nothing

[nalimilan]

The return type f(1) used to be inferred as Int on 0.6.2, but on master it's inferred as Union{Int,Nothing}. It still works when using isa Nothing though.

f(x) = x === nothing ? nothing : 1

[JeffBezanson]

Maybe related to #26339?

[JeffBezanson]

I tried this patch, which much to my dismay does not work:

--- a/base/compiler/abstractinterpretation.jl
+++ b/base/compiler/abstractinterpretation.jl
@@ -546,13 +546,15 @@ function abstract_call(@nospecialize(f), fargs::Union{Tuple{},Vector{Any}}, argt
                 # if doing a comparison to a singleton, consider returning a `Conditional` instead
                 if isa(aty, Const) && isa(b, Slot)
                     if isdefined(typeof(aty.val), :instance) # can only widen a if it is a singleton
-                        return Conditional(b, aty, typesubtract(widenconst(bty), typeof(aty.val)))
+                        trueT = (isa(rt,Const) && rt.val === false) ? Bottom : aty
+                        return Conditional(b, trueT, typesubtract(widenconst(bty), typeof(aty.val)))
                     end
                     return isa(rt, Const) ? rt : Conditional(b, aty, bty)
                 end
                 if isa(bty, Const) && isa(a, Slot)
                     if isdefined(typeof(bty.val), :instance) # same for b
-                        return Conditional(a, bty, typesubtract(widenconst(aty), typeof(bty.val)))
+                        trueT = (isa(rt,Const) && rt.val === false) ? Bottom : bty
+                        return Conditional(a, trueT, typesubtract(widenconst(aty), typeof(bty.val)))
                     end
                     return isa(rt, Const) ? rt : Conditional(a, bty, aty)
                 end

It fails during sysimg build with:

error during bootstrap:
LoadError("sysimg.jl", 445, LoadError("docs/basedocs.jl", 10, MethodError(convert, (String, 0x0000000000000058), 0x00000000000038bc)))
rec_backtrace at /home/bezanson/src/julia/src/stackwalk.c:94
record_backtrace at /home/bezanson/src/julia/src/task.c:246
jl_throw at /home/bezanson/src/julia/src/task.c:577
jl_method_error_bare at /home/bezanson/src/julia/src/gf.c:1608
jl_method_error at /home/bezanson/src/julia/src/gf.c:1626
jl_lookup_generic_ at /home/bezanson/src/julia/src/gf.c:2117 [inlined]
jl_apply_generic at /home/bezanson/src/julia/src/gf.c:2137
convert at ./essentials.jl:248
convert at ./some.jl:21
unknown function (ip: 0x7fb6fd33fc84)
jl_fptr_trampoline at /home/bezanson/src/julia/src/gf.c:1823
jl_apply_generic at /home/bezanson/src/julia/src/gf.c:2140
setproperty! at ./sysimg.jl:19
unknown function (ip: 0x7fb6fd33fc4b)
jl_fptr_trampoline at /home/bezanson/src/julia/src/gf.c:1823
jl_apply_generic at /home/bezanson/src/julia/src/gf.c:2140
popfirst! at ./iterators.jl:1078 [inlined]
iterate at ./iterators.jl:1085 [inlined]
iterate at ./iterators.jl:1085 [inlined]
join at ./strings/io.jl:246
unknown function (ip: 0x7fb6fd33f99d)
jl_fptr_trampoline at /home/bezanson/src/julia/src/gf.c:1823
jl_apply_generic at /home/bezanson/src/julia/src/gf.c:2140
print at ./version.jl:77
#print_to_string#337 at ./strings/io.jl:122
jl_fptr_trampoline at /home/bezanson/src/julia/src/gf.c:1823
jl_apply_generic at /home/bezanson/src/julia/src/gf.c:2140
print_to_string at ./strings/io.jl:110 [inlined]
string at ./strings/io.jl:141

Interestingly, even just disabling the code to return Conditional for calls to === also fails, which makes me think something else somewhere is very wrong.

[JeffBezanson]

If I disable the Conditional code for === I get this:

julia: /home/jeff/src/julia/deps/srccache/llvm-6.0.0/include/llvm/Support/Casting.h:106: static bool llvm::isa_impl_cl<To, const From*>::doit(const From*) [with To = llvm::BranchInst; From = llvm::TerminatorInst]: Assertion `Val && "isa<> used on a null pointer"' failed.

signal (6): Aborted
in expression starting at show.jl:644
raise at /build/buildd/eglibc-2.19/signal/../nptl/sysdeps/unix/sysv/linux/raise.c:56
abort at /build/buildd/eglibc-2.19/stdlib/abort.c:89
__assert_fail_base at /build/buildd/eglibc-2.19/assert/assert.c:92
__assert_fail at /build/buildd/eglibc-2.19/assert/assert.c:101
_ZN4llvm11isa_impl_clINS_10BranchInstEPKNS_14TerminatorInstEE4doitES4_.part.294 at /home/jeff/src/julia/usr/bin/../lib/libLLVM-6.0.so (unknown line)
_ZN4llvm14GetIfConditionEPNS_10BasicBlockERS1_S2_ at /home/jeff/src/julia/usr/bin/../lib/libLLVM-6.0.so (unknown line)
_ZL19FoldTwoEntryPHINodePN4llvm7PHINodeERKNS_19TargetTransformInfoERKNS_10DataLayoutE at /home/jeff/src/julia/usr/bin/../lib/libLLVM-6.0.so (unknown line)
_ZN4llvm11simplifyCFGEPNS_10BasicBlockERKNS_19TargetTransformInfoERKNS_18SimplifyCFGOptionsEPNS_15SmallPtrSetImplIS1_EE at /home/jeff/src/julia/usr/bin/../lib/libLLVM-6.0.so (unknown line)
_ZL22iterativelySimplifyCFGRN4llvm8FunctionERKNS_19TargetTransformInfoERKNS_18SimplifyCFGOptionsE at /home/jeff/src/julia/usr/bin/../lib/libLLVM-6.0.so (unknown line)
_ZN12_GLOBAL__N_115CFGSimplifyPass13runOnFunctionERN4llvm8FunctionE at /home/jeff/src/julia/usr/bin/../lib/libLLVM-6.0.so (unknown line)
_ZN4llvm13FPPassManager13runOnFunctionERNS_8FunctionE at /home/jeff/src/julia/usr/bin/../lib/libLLVM-6.0.so (unknown line)
_ZN4llvm13FPPassManager11runOnModuleERNS_6ModuleE at /home/jeff/src/julia/usr/bin/../lib/libLLVM-6.0.so (unknown line)
_ZN4llvm6legacy15PassManagerImpl3runERNS_6ModuleE at /home/jeff/src/julia/usr/bin/../lib/libLLVM-6.0.so (unknown line)
operator() at /home/jeff/src/julia/src/jitlayers.cpp:485
addModule at /home/jeff/src/julia/usr/include/llvm/ExecutionEngine/Orc/IRCompileLayer.h:57 [inlined]