inference: improve isdefined_tfunc accuracy for MustAlias (#58743)

Author: aviatesk

Compiler/src/tfuncs.jl

@@ -405,6 +405,9 @@ end
     return isdefined_tfunc(𝕃, arg1, sym)
 end
 @nospecs function isdefined_tfunc(𝕃::AbstractLattice, arg1, sym)
+    if arg1 isa MustAlias
+        arg1 = widenmustalias(arg1)
+    end
     arg1t = arg1 isa Const ? typeof(arg1.val) : isconstType(arg1) ? typeof(arg1.parameters[1]) : widenconst(arg1)
     a1 = unwrap_unionall(arg1t)
     if isa(a1, DataType) && !isabstracttype(a1)

Compiler/test/inference.jl

@@ -2313,12 +2313,6 @@ let 𝕃ᵢ = InferenceLattice(MustAliasesLattice(BaseInferenceLattice.instance)
     @test ifelse_tfunc(MustAlias(2, AliasableField{Any}, 1, Int), Int, Int) === Union{}
 end
 
-@testset "issue #56913: `BoundsError` in type inference" begin
-    R = UnitRange{Int}
-    @test Type{AbstractVector} == Base.infer_return_type(Base.promote_typeof, Tuple{R, R, Vector{Any}, Vararg{R}})
-    @test Type{AbstractVector} == Base.infer_return_type(Base.promote_typeof, Tuple{R, R, Vector{Any}, R, Vararg{R}})
-end
-
 maybeget_mustalias_tmerge(x::AliasableField) = x.f
 maybeget_mustalias_tmerge(x) = x
 @test Base.return_types((Union{Nothing,AliasableField{Any}},); interp=MustAliasInterpreter()) do x
@@ -2593,6 +2587,19 @@ end |> only === Compiler.InterMustAlias
     return 0
 end == Integer
 
+# `isdefined` accuracy for `MustAlias`
+@test Base.infer_return_type((Any,); interp=MustAliasInterpreter()) do x
+    xx = Ref{Any}(x)
+    xxx = Some{Any}(xx)
+    Val(isdefined(xxx.value, :x))
+end == Val{true}
+
+@testset "issue #56913: `BoundsError` in type inference" begin
+    R = UnitRange{Int}
+    @test Type{AbstractVector} == Base.infer_return_type(Base.promote_typeof, Tuple{R, R, Vector{Any}, Vararg{R}})
+    @test Type{AbstractVector} == Base.infer_return_type(Base.promote_typeof, Tuple{R, R, Vector{Any}, R, Vararg{R}})
+end
+
 function f25579(g)
     h = g[]
     t = (h === nothing)