make the default constructor type stable (#389)

Author: ranocha

src/abstract.jl

@@ -83,7 +83,9 @@ macro register(name)
         Base.promote_rule(::Type{<:$poly{T,X}}, ::Type{<:$poly{S,X}}) where {T,S,X} =  $poly{promote_type(T, S),X}
         Base.promote_rule(::Type{<:$poly{T,X}}, ::Type{S}) where {T,S<:Number,X} =
             $poly{promote_type(T, S),X}
-        $poly(coeffs::AbstractVector{T}, var::SymbolLike = :x) where {T} =
+        $poly(coeffs::AbstractVector{T}) where {T} =
+            $poly{T, :x}(coeffs)
+        $poly(coeffs::AbstractVector{T}, var::SymbolLike) where {T} =
             $poly{T, Symbol(var)}(coeffs)
         $poly{T}(x::AbstractVector{S}, var::SymbolLike = :x) where {T,S} =
             $poly{T,Symbol(var)}(T.(x))

test/StandardBasis.jl

@@ -393,6 +393,10 @@ end
         pN = P([276,3,87,15,24,0])
         pR = P([3 // 4, -2 // 1, 1 // 1])
 
+        # type stability of the default constructor without variable name
+        if P !== ImmutablePolynomial
+            @inferred P([1, 2, 3])
+        end
 
         @test p3 == P([1,2,1])
         @test pN * 10 == P([2760, 30, 870, 150, 240])