In program as follows function template f having two arguments declares the type of the second argument by using decltype with lambda expression inside referring to the first function’s argument p (here for illustration purposes it just removes the reference from the type):

int f( auto && p, 
    decltype([p](){return p;}()) ) //#1
    { return 0; }

int main() {
    int i = 0;
    return f( i, 0 );
}

Unfortunately, GCC rejects it with the error:

error: use of parameter outside function body before ‘;’ token

MSVC:

error C3536: ‘p’: cannot be used before it is initialized

Clang’s error is less clear:

error: variable ‘p’ cannot be implicitly captured in a lambda with no capture-default specified

Online demo: https://gcc.godbolt.org/z/oeWMb6ozo

Now if one uses an initializer for lambda capture:

int f( auto && p, 
    decltype([p=p](){return p;}()) ) //#2
    { return 0; }

the behavior of compilers does change.

MSVC:

error C2853: ‘p’: a non-static data member cannot have a type that contains ‘auto’

Clang simply crashes.

But GCC accepts program #2 just fine. Online demo: https://gcc.godbolt.org/z/ebco5cYsh

Is program #2 really well-formed and #1 not? What are the limitations on using function template arguments for defining types of other arguments?