fix CG test, add some type constraints for clarity

Author: mateuszbaran

src/plans/stepsize/stepsize.jl

@@ -74,11 +74,11 @@ mutable struct ArmijoLinesearchStepsize{TRM <: AbstractRetractionMethod, P, I, F
             initial_guess::IGF = ArmijoInitialGuess(),
             retraction_method::TRM = default_retraction_method(M),
             stop_when_stepsize_less::F = 0.0,
-            stop_when_stepsize_exceeds = max_stepsize(M),
+            stop_when_stepsize_exceeds::Real = max_stepsize(M),
             stop_increasing_at_step::I = 100,
             stop_decreasing_at_step::I = 1000,
             sufficient_decrease = 0.1,
-        ) where {TRM, P, I, F, IGF, DF, IF}
+        ) where {TRM <: AbstractRetractionMethod, P, I, F <: Real, IGF <: AbstractInitialLinesearchGuess, DF, IF}
         msgs = (;
             non_descent_direction = StepsizeMessage{F, F}(),
             stop_decreasing = StepsizeMessage{Int, F}(),

test/solvers/test_conjugate_gradient.jl

@@ -327,7 +327,7 @@ using LinearAlgebra: Diagonal, dot, eigvals, eigvecs
 
         stopping_criterion = StopAfterIteration(30)
         get_stepsize() = Manopt.ArmijoLinesearchStepsize(
-            M; initial_stepsize = 1.0, initial_guess = (args...) -> 1.0
+            M; initial_stepsize = 1.0, initial_guess = Manopt.ConstantInitialGuess(1.0)
         )
 
         p1 = conjugate_gradient_descent(