Compiler bug with `@ccall` and parametric types

[mattsignorelli]

Following from this discussion on the discourse

A summary of the problem is:

# The following works fine:
mt = MyType{Int32,CTable(16)}() 
# Define convenience ctor:
MyType{T}() where {T} = MyType{T,CTable(16)}()
# This throws a weird error:
MType{Int32}() 

Here is a full minimal working example of the problem:

struct CTable 
  t::Ptr{Float64} 

  function CTable(n)
    # In practice t is received from the C program
    t = @ccall malloc(n::Csize_t)::Ptr{Float64}
    return new(t)
  end
end

mutable struct MyType{T,Table}
  arr::Ptr{T}

  function MyType{T,Table}() where {T,Table}
    arr = @ccall jl_malloc(10::Csize_t)::Ptr{T}
    m = new{T,Table}(arr)
    f(m) = @ccall jl_free(m.arr::Ptr{Cvoid})::Cvoid
    finalizer(f, m)
    return m
  end
end

# The following works fine:
mt = MyType{Int32,CTable(16)}() 

# However this convenience constructor breaks the code:
MyType{T}() where {T} = MyType{T,CTable(16)}()

MyType{Int32}() # Error!!!
#=
ERROR: ccall return type Ptr should have an element type (not Ptr{<:T})
Stacktrace:
 [1] MyType
   @ ./REPL[2]:5 [inlined]
 [2] (MyType{Int32})()
   @ Main ./REPL[4]:1
 [3] top-level scope
   @ REPL[6]:1
=#

Note that this works fine if one of the following:

1. The type is defined without a second parametric type Table
2. The "convenience ctor" is fully defined separately without calling the inner ctor
3. @ccall specifies a return type Ptr{Cvoid}, and then unsafe_converts to Ptr{T}

The underlying problem was then found by @Seelengrab as discussed at the end of the discourse post

[Keno]

Type parameters in ccall signatures are only partially supported. The reason for that is that (as in this case) it's usually incorrect to use them there. In particular, the signature of jl_malloc does not change. It always returns a Ptr{Void} that them needs to be casted. The types in a ccall signature need to match the c declaration exactly - everything else is UB.

[vtjnash]

Note that this example ran correctly before inlining, then there was a bug introduced to make the example crash