inference: fix #42090, make sure not to wrap Conditional in PartialStruct (#42091)

(cherry picked from commit 03e7b23)

Author: aviatesk

base/compiler/abstractinterpretation.jl

@@ -1190,7 +1190,7 @@ function abstract_eval_statement(interp::AbstractInterpreter, @nospecialize(e),
             anyconst = false
             allconst = true
             for i = 2:length(e.args)
-                at = abstract_eval_value(interp, e.args[i], vtypes, sv)
+                at = widenconditional(abstract_eval_value(interp, e.args[i], vtypes, sv))
                 if !anyconst
                     anyconst = has_nontrivial_const_info(at)
                 end

test/compiler/inference.jl

@@ -1719,6 +1719,29 @@ for expr25261 in opt25261[i:end]
 end
 @test foundslot
 
+# https://github.com/JuliaLang/julia/issues/42090#issuecomment-911824851
+# `PartialStruct` shoudln't wrap `Conditional`
+let M = Module()
+    @eval M begin
+        struct BePartialStruct
+            val::Int
+            cond
+        end
+    end
+
+    rt = @eval M begin
+        Base.return_types((Union{Nothing,Int},)) do a
+            cond = a === nothing
+            obj = $(Expr(:new, M.BePartialStruct, 42, :cond))
+            r1 = getfield(obj, :cond) ? 0 : a # r1::Union{Nothing,Int}, not r1::Int (because PartialStruct doesn't wrap Conditional)
+            a = $(gensym(:anyvar))::Any
+            r2 = getfield(obj, :cond) ? a : nothing # r2::Any, not r2::Const(nothing) (we don't need to worry about constrait invalidation here)
+            return r1, r2 # ::Tuple{Union{Nothing,Int},Any}
+        end |> only
+    end
+    @test rt == Tuple{Union{Nothing,Int},Any}
+end
+
 function f25579(g)
     h = g[]
     t = (h === nothing)