Can't compile SingleValueGenerator inside GENERATE_COPY

[TysonRayJones]

Describe the bug
Attempting to invoke the SingleValueGenerator (using external variables) inside GENERATE_COPY like so...

int a = 1;
int b = 2;
int c = GENERATE_COPY( a,b );

results in a compilation error:

error: no matching function for call to 'Catch::Generators::GeneratorWrapper<const int&>::GeneratorWrapper(Catch::Generators::GeneratorWrapper<const int>)'
  { ::new((void *)__p) _Up(std::forward<_Args>(__args)...); }

Expected behavior
The above example code should compile fine, since GENERATE_COPY should allow the use of external variables (a and b) in any generator. For example, the following code compiles totally fine (using RangeGenerator):

int a = 1;
int b = 2;
int c = GENERATE_COPY( range(a,b) );

Reproduction steps
self-contained code bugdemo.cpp:

#include "catch.hpp"

TEST_CASE( "bug " ) {
    
    int a = 1;
    int b = 2;
    int c = GENERATE_COPY(a, b);
    REQUIRE( true );
}

and compiled simply with

g++ -std=c++14 -c bugdemo.cpp

Platform information:

• OS: MacOS 10.14 and Ubuntu 18.04
• Compiler+version: GCC v8.2.0 and GCC v7.4.0 and clang v10.0.0 (and all other versions on system)
• Catch version: v2.10.0

[TysonRayJones]

Lovely, thank you very much!