Missing optimization: if constexpr(false) prevents NRVO

[apolukhin]

Bugzilla Link	50953
Version	trunk
OS	Linux
CC	@apolukhin,@zygoloid

Extended Description

Consider the following code:

struct A {
A() = default;
A(A&&);
};

template
A foo() {
A a;
if constexpr (B) {
return A();
}
return a;
}

A test() {
return foo();
}

In the above example NRVO for a is applied by many compilers, but not clang.

Godbolt playground: https://godbolt.org/z/P8G3zj5GE

[Izaron]

I've spent some time researching this problem.

The problem is that VarDecls are checked for nrvo attribute once and for all, reading the source code "as written".
That is, a VarDecl is either non-nrvo or nrvo in all template instantiations.

ConstexprCondition has three possible values:

1. if constexpr (false) {...} - the statement is discarded
2. if constexpr (true) {...} - the statement is not discarded
3. if constexpr (B) {...} - the statement may be or be not discarded, but as clang doesn't check it in every instantiation, it works as the statement is not discarded.

I think that the solution is to calculate nrvo VarDecls on a "lower" level, working on the prepared AST instead of sema::Scopes.
But this kind of task is not an easy fix, and I guess it is unbearable for a casual contributor.