fix #41489: inference of +(::Rational, Rational) (#41491)

* fix #41489: inference of `+(::Rational, Rational)`

* implement review comments

(cherry picked from commit cf4e1c4)

Author: simeonschaub

base/rational.jl

@@ -280,7 +280,7 @@ function -(x::Rational{T}) where T<:Unsigned
 end
 
 function +(x::Rational, y::Rational)
-    xp, yp = promote(x, y)
+    xp, yp = promote(x, y)::NTuple{2,Rational}
     if isinf(x) && x == y
         return xp
     end
@@ -289,7 +289,7 @@ function +(x::Rational, y::Rational)
 end
 
 function -(x::Rational, y::Rational)
-    xp, yp = promote(x, y)
+    xp, yp = promote(x, y)::NTuple{2,Rational}
     if isinf(x) && x == -y
         return xp
     end

test/rational.jl

@@ -622,3 +622,11 @@ end
 @testset "Rational{T} with non-concrete T (issue #41222)" begin
     @test @inferred(Rational{Integer}(2,3)) isa Rational{Integer}
 end
+
+@testset "issue #41489" begin
+    @test Core.Compiler.return_type(+, NTuple{2, Rational}) == Rational
+    @test Core.Compiler.return_type(-, NTuple{2, Rational}) == Rational
+
+    A=Rational[1 1 1; 2 2 2; 3 3 3]
+    @test @inferred(A*A) isa Matrix{Rational}
+end