sizeof with function template argument in declaration doesn't match the definition

The following program

template<int> 
struct R{};

struct S
{
    template<typename T>
    auto f(T t) -> R<sizeof(t)>;
};

template<typename T>
auto S::f(T t) -> R<sizeof(t)> {}

compiles with Clang, but GCC 10.2 gives an error for the definition of f

error: no declaration matches 'R<sizeof (t)> S::f(T)'

This looks like a GCC bug, especially since GCC trunk currently gives an ICE

internal compiler error: canonical types differ for identical types 'R<sizeof (t)>' and 'R<sizeof (t)>'

Also, GCC compiles the program when the return type in both the declaration and the definition is changed to R<sizeof (T)>.

Is the program valid?

Here's the demo.