fix "improved" correctness of fieldtype

the getfield_tfunc was missing a test for whether
the fields were all equivalent before concluding
that the result type was exact

fixes the fix JuliaLang#17953
fix JuliaLang#18037

base/inference.jl

@@ -479,7 +479,7 @@ function getfield_tfunc(s0::ANY, name)
  # in the current type system
  typ = limit_type_depth(R, 0, true,
  filter!(x->isa(x,TypeVar), Any[s.parameters...]))
- return typ, isleaftype(s) && typeseq(typ, R)
+ return typ, isleaftype(s) && isa(R, Type) && typeof(R) === typeof(typ) && typeseq(R, typ)
  end
  end
  end
@@ -502,14 +502,15 @@ function getfield_tfunc(s0::ANY, name)
  elseif length(s.types) == 1 && isempty(s.parameters)
  return s.types[1], true
  else
- R = reduce(tmerge, Bottom, map(unwrapva,s.types)) #=Union{s.types...}=#
+ R = reduce(tmerge, Bottom, map(unwrapva, s.types)) #=Union{s.types...}=#
+ alleq = isa(R, Type) && typeof(R) === typeof(s.types[1]) && typeseq(R, s.types[1])
  # do the same limiting as the known-symbol case to preserve type-monotonicity
  if isempty(s.parameters)
- return R, typeseq(R, s.types[1])
+ return R, alleq
  else
  typ = limit_type_depth(R, 0, true,
  filter!(x->isa(x,TypeVar), Any[s.parameters...]))
- return typ, isleaftype(s) && typeseq(typ, R)
+ return typ, alleq && isleaftype(s) && typeof(R) === typeof(typ) && typeseq(R, typ)
  end
  end
 end

test/inference.jl

@@ -307,6 +307,21 @@ let T = Array{Tuple{Vararg{Float64,TypeVar(:dim)}},1},
 end
 @test f16530a(:d) == Vector
 
+let T1 = Tuple{Int, Float64},
+ T2 = Tuple{Int, Float32},
+ T = Tuple{T1, T2}
+
+ global f18037
+ f18037() = fieldtype(T, 1)
+ f18037(i) = fieldtype(T, i)
+
+ @test f18037() === T1
+ @test f18037(1) === T1
+ @test f18037(2) === T2
+
+ @test Base.return_types(f18037, ()) == Any[Type{T1}]
+ @test Base.return_types(f18037, (Int,)) == Any[Type{TypeVar(:T, Tuple{Int, AbstractFloat})}]
+end
 
 # issue #18015
 type Triple18015