Fix promote_operation for * with rationals (#261)

* Fix promote_operation for * with rationals

* Fix format

Author: blegat

src/promote.jl

@@ -300,3 +300,30 @@ function MA.promote_operation(
 ) where {T,P<:_APL}
     return promote_operation_constant(*, P, T)
 end
+
+function MA.promote_operation(
+    ::typeof(*),
+    ::Type{P},
+    ::Type{RationalPoly{NT,DT}},
+) where {P<:APL,NT,DT}
+    return RationalPoly{MA.promote_operation(*, P, NT),DT}
+end
+
+function MA.promote_operation(
+    ::typeof(*),
+    ::Type{RationalPoly{NT,DT}},
+    ::Type{P},
+) where {P<:APL,NT,DT}
+    return RationalPoly{MA.promote_operation(*, NT, P),DT}
+end
+
+function MA.promote_operation(
+    ::typeof(*),
+    ::Type{RationalPoly{NS,DS}},
+    ::Type{RationalPoly{NT,DT}},
+) where {NS,DS,NT,DT}
+    return RationalPoly{
+        MA.promote_operation(*, NS, NT),
+        MA.promote_operation(*, DS, DT),
+    }
+end

test/promote.jl

@@ -118,3 +118,34 @@ Base.:+(::C, ::C) = C()
     @test MA.promote_operation(*, A, polynomial_type(x, A)) ==
           polynomial_type(x, B)
 end
+
+function __promote_prod(::Type{A}, ::Type{B}, ::Type{C}) where {A,B,C}
+    @test MA.promote_operation(*, A, B) == C
+    @test MA.promote_operation(*, B, A) == C
+end
+
+@testset "promote_operation with Rational" begin
+    Mod.@polyvar x
+    V = typeof(x)
+    M = monomial_type(V)
+    T = term_type(V, Int)
+    P = polynomial_type(V, Float64)
+    function _promote_prod(::Type{A}, ::Type{B}, ::Type{C}) where {A,B,C}
+        __promote_prod(A, B, C)
+        __promote_prod(RationalPoly{A,B}, RationalPoly{B,A}, RationalPoly{C,C})
+        __promote_prod(RationalPoly{A,A}, RationalPoly{B,B}, RationalPoly{C,C})
+        for U in [V, M, T, P]
+            __promote_prod(A, RationalPoly{B,U}, RationalPoly{C,U})
+        end
+    end
+    _promote_prod(V, V, M)
+    for U in [V, M]
+        _promote_prod(U, M, M)
+    end
+    for U in [V, M, T]
+        _promote_prod(U, T, T)
+    end
+    for U in [V, M, T, P]
+        _promote_prod(U, P, P)
+    end
+end