StackOverflow in function definition

[blegat]

The following works fine with Julia v1.0 both for defining the function and resolving which method should be called but it fails in Julia v1.1 as follows:

julia> versioninfo()
Julia Version 1.1.0
Commit 80516ca202 (2019-01-21 21:24 UTC)
Platform Info:
  OS: Linux (x86_64-pc-linux-gnu)
  CPU: Intel(R) Core(TM) i7-6700HQ CPU @ 2.60GHz
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-6.0.1 (ORCJIT, skylake)

julia> struct A{B, C} end

julia> f(::A{B, C}, ::Union{C, A{B, C}}, ::Union{B, Real}) where {B, C} = 0
f (generic function with 1 method)

julia> f(::A{B, C}, ::C, ::C) where {B, C} = 1
ERROR: StackOverflowError:
Stacktrace:
 [1] top-level scope at none:0

[JeffBezanson]

Looks like potentially the same issue as #30741