[clang, trunk, c++11] Clang rejects NTTP defaulted value invoking static constexpr member function within class

[CalebAdrianXYZ]

Given the following C++11 code:

// minimal reproducible example, oversimplified code
struct foo {
    static constexpr bool bar() {
        return true;
    }

    template<bool B = bar()> // <-- clang has issue with this
    static constexpr bool baz() {
        return true;
    }
};
static_assert(foo::baz(), "");

GCC, MSVC, ICC, and NVC++ all compile this code successfully, but Clang (trunk) produces the following diagnostic output:

<source>:7:23: error: non-type template argument is not a constant expression
    template<bool B = bar()> // <-- clang has issue with this
                      ^~~~~
<source>:7:23: note: undefined function 'bar' cannot be used in a constant expression
<source>:3:27: note: declared here
    static constexpr bool bar() {
                          ^
<source>:12:15: error: no matching function for call to 'baz'
static_assert(foo::baz(), "");
              ^~~~~~~~
2 errors generated.
Compiler returned: 1

Corresponding CE link: https://godbolt.org/z/4Md1b5o8r

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[shafik]

@cor3ntin is this #62462

[tbaederr]

In

llvm-project/clang/lib/AST/ExprConstant.cpp

Lines 7802 to 7808 in 231ef21

	const FunctionDecl *Definition = nullptr;
	Stmt *Body = FD->getBody(Definition);
	if (!CheckConstexprFunction(Info, E->getExprLoc(), FD, Definition, Body) ||
	!HandleFunctionCall(E->getExprLoc(), Definition, This, Args, Call,
	Body, Info, Result, ResultSlot))
	return false;

both Body and Definition are nullptr, so the following CheckConstexprFunction diagnoses.

I don't know enough about the parsing here, are the bodies of static member functions only available later?

[cor3ntin]

@cor3ntin is this #62462

Nope, but it might be #62224
Ie, we may be able to evaluate the argument too soon,
That or we don't do late parsing of template default arguments, which certainly would be surprising

[Fznamznon]

That feels similar to #62224, although bar() is defined at the point we should be parsing baz(). Let me take a look.

[Fznamznon]

https://reviews.llvm.org/D150108 that I was originally intending for #62224 helps here as well.