Add better error messages for tangent accessors when types are passed

src/fwds_rvs_data.jl

@@ -258,6 +258,14 @@ function fdata(t::T) where {T}
     return F(fdata(t.fields))
 end
 
+function fdata(::Type{T}) where {T}
+    throw(
+        error(
+            "$T is a type. Perhaps you meant fdata_type($T) or fdata(instance_of_tangent)?"
+        ),
+    )
+end
+
 function fdata(t::T) where {T<:PossiblyUninitTangent}
     F = fdata_type(T)
     return is_init(t) ? F(fdata(val(t))) : F()
@@ -530,6 +538,14 @@ function rdata(t::T) where {T}
     return R(rdata(t.fields))
 end
 
+function rdata(::Type{T}) where {T}
+    throw(
+        error(
+            "$T is a type. Perhaps you meant rdata_type($T) or rdata(instance_of_tangent)?"
+        ),
+    )
+end
+
 function rdata(t::T) where {T<:PossiblyUninitTangent}
     R = rdata_type(T)
     return is_init(t) ? R(rdata(val(t))) : R()

test/fwds_rvs_data.jl

@@ -157,4 +157,41 @@ end
             @test verify_rdata_value(Ptr{Float64}(), NoRData()) === nothing
         end
     end
+
+    @testset "Error messages for types passed to accessor functions" begin
+        # Test that passing types to fdata throws helpful error message
+        @test_throws "Float64 is a type. Perhaps you meant fdata_type(Float64) or fdata(instance_of_tangent)?" fdata(
+            Float64
+        )
+        @test_throws "Int64 is a type. Perhaps you meant fdata_type(Int64) or fdata(instance_of_tangent)?" fdata(
+            Int64
+        )
+
+        # Test that passing types to rdata throws helpful error message  
+        @test_throws "Float64 is a type. Perhaps you meant rdata_type(Float64) or rdata(instance_of_tangent)?" rdata(
+            Float64
+        )
+        @test_throws "Int64 is a type. Perhaps you meant rdata_type(Int64) or rdata(instance_of_tangent)?" rdata(
+            Int64
+        )
+
+        # Test with more complex types - using exact string matching since the message is deterministic
+        @test_throws "Vector{Float64} is a type. Perhaps you meant fdata_type(Vector{Float64}) or fdata(instance_of_tangent)?" fdata(
+            Vector{Float64}
+        )
+        @test_throws "Vector{Float64} is a type. Perhaps you meant rdata_type(Vector{Float64}) or rdata(instance_of_tangent)?" rdata(
+            Vector{Float64}
+        )
+
+        # Verify that existing functionality with instances still works
+        x = 5.0
+        t = zero_tangent(x)
+        @test fdata(t) isa NoFData
+        @test rdata(t) isa Float64
+
+        arr = [1.0, 2.0]
+        t_arr = zero_tangent(arr)
+        @test fdata(t_arr) isa Vector{Float64}
+        @test rdata(t_arr) isa NoRData
+    end
 end