Type inference of constructors with known marginals

[mschauer]

In the following example, the constructor gets inferred as returning T{A, B}, and not T{Int, B} and the known type of the a-coordinate gets lost .

type T{A, B}
    a::A
    b::B
end
println(code_typed(T, (Int, Any)))

[vtjnash]

can you explain why this is a problem? actual dispatch on the type signature (Int,Any) is impossible since Any is not a concrete leaf type

[mschauer]

A hopefully not too contrieved example, where f should be typestable

immutable BaseKVector{K, T}
    x::Vector{T}
end
BaseKVector{T}(v::Vector{T}, K) = BaseKVector{K, T}(v)
BaseKVector{K1,T}(v::BaseKVector{K1,T}, K2) = BaseKVector{K2, T}(v.x)
indexrange{K}(v::BaseKVector{K}) = K : length(v.x) + K

f(x) = indexrange(BaseKVector(x, 1))

println(code_typed(BaseKVector, (BaseKVector, Int)))
println(code_typed(f, (Vector{Int},)))

[JeffBezanson]

Similar to #5560, but more expensive since I think it would depend on re-running type inference after inlining. The problem is that, for obvious reasons, we have to generally avoid specializing functions for different values of arguments.

[JeffBezanson]

The example in the OP is fixed now. We should add a test.