constant propagation in Space power (#360)

* constant propagation in Space power

* version limit tests

Author: jishnub

src/Multivariate/TensorSpace.jl

@@ -345,14 +345,21 @@ Space(sp::ProductDomain) = TensorSpace(sp)
 setdomain(sp::TensorSpace, d::ProductDomain) = TensorSpace(setdomain.(factors(sp), factors(d)))
 
 *(A::Space, B::Space) = A ⊗ B
-function ^(A::Space, p::Integer)
+@inline function _powspace(A, p)
     p >= 1 || throw(ArgumentError("exponent must be >= 1, received $p"))
     # Enumerate common cases to help with constant propagation
     p == 1 ? A :
     p == 2 ? A * A :
     p == 3 ? A * A * A :
     foldl(*, ntuple(_ -> A, p))
 end
+@static if VERSION >= v"1.8"
+    Base.@constprop :aggressive function ^(A::Space, p::Integer)
+        _powspace(A, p)
+    end
+else
+    ^(A::Space, p::Integer) = _powspace(A, p)
+end
 
 
 ## TODO: generalize

test/SpacesTest.jl

@@ -71,9 +71,15 @@ using LinearAlgebra
             end
             @test ApproxFunBase.intpow(f,-2) == f^-2 == 1/(f*f)
 
-            @test sp^1 == sp
-            @test sp^2 == sp * sp
-            @test sp^3 == sp * sp * sp
+            if VERSION >= v"1.8"
+                @test (@inferred (x -> x^1)(sp)) == sp
+                @test (@inferred (x -> x^2)(sp)) == sp * sp
+                @test (@inferred (x -> x^3)(sp)) == sp * sp * sp
+            else
+                @test sp^1 == sp
+                @test sp^2 == sp * sp
+                @test sp^3 == sp * sp * sp
+            end
             @test sp^4 == sp * sp * sp * sp
             @test sp^5 == sp * sp * sp * sp * sp
         end