Workaround for promote_type for GPU arrays

src/LinearOperatorCollection.jl

@@ -108,4 +108,35 @@ include("SamplingOp.jl")
 include("NormalOp.jl")
 include("DiagOp.jl")
 
+function promote_storage_types(A, B)
+  A_type = storage_type(A)
+  B_type = storage_type(B)
+  S = promote_type(A_type, B_type)
+  if !isconcretetype(S)
+    # Find common eltype
+    elType = promote_type(eltype(A), eltype(B))
+    if !isconcretetype(elType)
+      throw(LinearOperatorException("Storage types cannot be promoted to a concrete type"))
+    end
+
+    # Same base type
+    A_base = Base.typename(A_type).wrapper
+    B_base = Base.typename(B_type).wrapper
+    if A_base != B_base
+      throw(LinearOperatorException("Storage types cannot be promoted to a common base type"))
+    end
+
+    # LinearOperators only accepts DataTypes, so we cant just do A_base{elType}, since that might be a UnionAll
+    # Check if either A_type or B_type have the fitting eltype
+    if eltype(A_type) == elType
+      S = A_type
+    elseif eltype(B_type) == elType
+      S = B_type
+    else
+      throw(LinearOperatorException("Storage types cannot be promoted to a common eltype"))
+    end
+  end
+  return S
+end
+
 end

src/NormalOp.jl

@@ -52,8 +52,7 @@ end
 LinearOperators.storage_type(op::NormalOpImpl) = typeof(op.Mv5)
 
 function NormalOpImpl(parent, weights)
-  S = promote_type(storage_type(parent), storage_type(weights))
-  isconcretetype(S) || throw(LinearOperatorException("Storage types cannot be promoted to a concrete type"))
+  S = promote_storage_types(parent, weights)
   tmp = S(undef, size(parent, 1))
   return NormalOpImpl(parent, weights, tmp)
 end

src/ProdOp.jl

@@ -36,8 +36,7 @@ composition/product of two Operators. Differs with * since it can handle normal
 function ProdOp(A, B)
   nrow = size(A, 1)
   ncol = size(B, 2)
-  S = promote_type(LinearOperators.storage_type(A), LinearOperators.storage_type(B))
-  isconcretetype(S) || throw(LinearOperatorException("Storage types cannot be promoted to a concrete type"))
+  S = promote_storage_types(A, B)
   tmp_ = S(undef, size(B, 1))
 
   function produ!(res, x::AbstractVector{T}, tmp) where T<:Union{Real,Complex}