GCC : "definition of BLAH is not in namespace enclosing" - c++

Question

I'm trying to figure out something in VCVRack.
In Template.hpp
using namespace rack;
...
struct SlowSliderWidget : public OpaqueWidget {
void draw(NVGcontext *vg) override;
};
then in MyWidget.cpp
#include "Template.hpp"
namespace rack {
void SlowSliderWidget::draw(NVGcontext *vg) {
nvgBeginPath(vg);
nvgRect(vg, 0, 0, 100, 100);
nvgFill(vg);
}
} // namespace rack
This gives me the following compiler error :
g++ -Wsuggest-override -std=c++11 -DSLUG=Template -fPIC -I../../include -I../../dep/include -DVERSION=0.6.0 -MMD -MP -g -O3 -march=nocona -ffast-math -fno-finite-math-only -Wall -Wextra -Wno-unused-parameter -DARCH_LIN -c -o build/src/MyWidget.cpp.o src/MyWidget.cpp
src/MyWidget.cpp:6:43: error: definition of ‘void SlowSliderWidget::draw(NVGcontext*)’ is not in namespace enclosing ‘SlowSliderWidget’ [-fpermissive]
void SlowSliderWidget::draw(NVGcontext *vg) {
^
../../compile.mk:64: recipe for target 'build/src/MyWidget.cpp.o' failed
Now, searching around I find various complaints about this type of error in GCC.
But all of them seem to be about attempts to redefine the meaning of the same class name in the same namespace.
But if I understand my C++ here, I am NOT trying to redefine the name. I declare the name SlowSliderWidget and the SlowSliderWidget::draw in the Template.hpp file and I simply try to define the function body of draw in the MyWidget.cpp file.
So why am I getting an error message? The wording seems to imply I'm not in the same namespace. But in both files I am in namespace rack. Aren't I?