show: Fix non-stacktrace show of toplevel MethodInstance

While we're here, also check before accessing the `uninferred` field.
Top-level MethodInstances should always have this, but the system
doesn't enforce this and it's rude for a `show` method to throw on
corrupted data (since that's what people use to debug that).

base/show.jl

@@ -1336,15 +1336,15 @@ end
 
 show(io::IO, l::Core.MethodInstance) = show_mi(io, l)
 
-function show_mi(io::IO, l::Core.MethodInstance, from_stackframe::Bool=false)
- def = l.def
+function show_mi(io::IO, mi::Core.MethodInstance, from_stackframe::Bool=false)
+ def = mi.def
  if isa(def, Method)
- if isdefined(def, :generator) && l === def.generator
+ if isdefined(def, :generator) && mi === def.generator
  print(io, "MethodInstance generator for ")
  show(io, def)
  else
  print(io, "MethodInstance for ")
- show_tuple_as_call(io, def.name, l.specTypes; qualified=true)
+ show_tuple_as_call(io, def.name, mi.specTypes; qualified=true)
  end
  else
  print(io, "Toplevel MethodInstance thunk")
@@ -1353,11 +1353,13 @@ function show_mi(io::IO, l::Core.MethodInstance, from_stackframe::Bool=false)
  # added by other means. But if it isn't, then we should try
  # to print a little more identifying information.
  if !from_stackframe
- di = mi.uninferred.debuginfo
- file, line = IRShow.debuginfo_firstline(di)
- file = string(file)
- line = isempty(file) || line < 0 ? "<unknown>" : "$file:$line"
- print(io, " from ", def, " starting at ", line)
+ if isdefined(mi, :uninferred)
+ di = mi.uninferred.debuginfo
+ file, line = IRShow.debuginfo_firstline(di)
+ file = string(file)
+ line = isempty(file) || line < 0 ? "<unknown>" : "$file:$line"
+ print(io, " from ", def, " starting at ", line)
+ end
  end
  end
 end

test/show.jl

@@ -2694,3 +2694,10 @@ let lowered = Meta.lower(Main, Expr(:let, Expr(:block), Expr(:block, Expr(:tople
  # Check that this gets printed as `_1 = 1` not `y = 1`
  @test contains(sprint(show, ci), "_1 = 1")
 end
+
+# Toplevel MethodInstance with undef :uninferred
+let topmi = ccall(:jl_new_method_instance_uninit, Ref{Core.MethodInstance}, ());
+ topmi.specTypes = Tuple{}
+ topmi.def = Main
+ @test contains(repr(topmi), "Toplevel MethodInstance")
+end