From d51b1d1afbd0232f6590d28adf2fd8aae2ca7a9d Mon Sep 17 00:00:00 2001
From: Jameson Nash <vtjnash@gmail.com>
Date: Fri, 3 Sep 2021 14:16:03 -0400
Subject: [PATCH] inference: bail from const-prop if const-prop fails

Otherwise we can can end up in an infinite cycle of attempting
const-prop, and that failing. Also handle Varargs in method-lookup
matching, which was the cause for which we could not compute the
const-prop signature.

Fixes #42097
---
 base/compiler/abstractinterpretation.jl |  4 +++
 base/compiler/inferenceresult.jl        | 34 +++++++++++++------------
 test/compiler/inference.jl              |  7 +++++
 3 files changed, 29 insertions(+), 16 deletions(-)

diff --git a/base/compiler/abstractinterpretation.jl b/base/compiler/abstractinterpretation.jl
index 2bb3943a3f83f..492afa9b8ec1c 100644
--- a/base/compiler/abstractinterpretation.jl
+++ b/base/compiler/abstractinterpretation.jl
@@ -546,6 +546,10 @@ function abstract_call_method_with_const_args(interp::AbstractInterpreter, resul
             end
         end
         inf_result = InferenceResult(mi, argtypes, va_override)
+        if !any(inf_result.overridden_by_const)
+            add_remark!(interp, sv, "[constprop] Could not handle constant info in matching_cache_argtypes")
+            return nothing
+        end
         frame = InferenceState(inf_result, #=cache=#:local, interp)
         frame === nothing && return nothing # this is probably a bad generated function (unsound), but just ignore it
         frame.parent = sv
diff --git a/base/compiler/inferenceresult.jl b/base/compiler/inferenceresult.jl
index 026b5286979cb..483e2f38d9ee8 100644
--- a/base/compiler/inferenceresult.jl
+++ b/base/compiler/inferenceresult.jl
@@ -16,30 +16,32 @@ end
 function matching_cache_argtypes(linfo::MethodInstance, given_argtypes::Vector, va_override::Bool)
     @assert isa(linfo.def, Method) # ensure the next line works
     nargs::Int = linfo.def.nargs
-    @assert length(given_argtypes) >= (nargs - 1)
     given_argtypes = anymap(widenconditional, given_argtypes)
-    if va_override || linfo.def.isva
+    isva = va_override || linfo.def.isva
+    if isva || isvarargtype(given_argtypes[end])
         isva_given_argtypes = Vector{Any}(undef, nargs)
-        for i = 1:(nargs - 1)
+        for i = 1:(nargs - isva)
             isva_given_argtypes[i] = argtype_by_index(given_argtypes, i)
         end
-        if length(given_argtypes) >= nargs || !isvarargtype(given_argtypes[end])
-            isva_given_argtypes[nargs] = tuple_tfunc(given_argtypes[nargs:end])
-        else
-            isva_given_argtypes[nargs] = tuple_tfunc(given_argtypes[end:end])
+        if isva
+            if length(given_argtypes) < nargs && isvarargtype(given_argtypes[end])
+                last = length(given_argtypes)
+            else
+                last = nargs
+            end
+            isva_given_argtypes[nargs] = tuple_tfunc(given_argtypes[last:end])
         end
         given_argtypes = isva_given_argtypes
     end
+    @assert length(given_argtypes) == nargs
     cache_argtypes, overridden_by_const = matching_cache_argtypes(linfo, nothing, va_override)
-    if nargs === length(given_argtypes)
-        for i in 1:nargs
-            given_argtype = given_argtypes[i]
-            cache_argtype = cache_argtypes[i]
-            if !is_argtype_match(given_argtype, cache_argtype, overridden_by_const[i])
-                # prefer the argtype we were given over the one computed from `linfo`
-                cache_argtypes[i] = given_argtype
-                overridden_by_const[i] = true
-            end
+    for i in 1:nargs
+        given_argtype = given_argtypes[i]
+        cache_argtype = cache_argtypes[i]
+        if !is_argtype_match(given_argtype, cache_argtype, overridden_by_const[i])
+            # prefer the argtype we were given over the one computed from `linfo`
+            cache_argtypes[i] = given_argtype
+            overridden_by_const[i] = true
         end
     end
     return cache_argtypes, overridden_by_const
diff --git a/test/compiler/inference.jl b/test/compiler/inference.jl
index 2b85fcbfc943e..9ad091f08e582 100644
--- a/test/compiler/inference.jl
+++ b/test/compiler/inference.jl
@@ -3496,3 +3496,10 @@ end
         end
         return x
     end) === Union{Int, Float64, Char}
+
+# issue #42097
+struct Foo42097{F} end
+Foo42097(f::F, args) where {F} = Foo42097{F}()
+Foo42097(A) = Foo42097(Base.inferencebarrier(+), Base.inferencebarrier(1)...)
+foo42097() = Foo42097([1]...)
+@test foo42097() isa Foo42097{typeof(+)}