What's going on here? Auto/constant expression/floating point

[xismn]

Hello,

Can someone explain why the following snippet is erronous?

namespace foo {
	const auto a = 2.0;
}

class Test {
public:
	Test() {}

	const static int bar = foo::a;

};

I'm receiving the following error for line 9:

error C2057: expected constant expression

The error is referring to foo::a. If I remove the .0 from foo::a's initialization (as in, it's an int, not a double), the snippet compiles fine.

Does this possibly somehow have to do with non-integral types being disallowed from having in-class initializers? However, Test::bar is an integral type...

[JLBorges]

Perform the conversion from double to int at compile-time (constexpr).

namespace foo {
    constexpr auto a = 2.0;
}

http://coliru.stacked-crooked.com/a/d4ebb9a7dea036cc

Note: Since this International Standard imposes no restrictions on the accuracy of floating-point operations, it is unspecified whether the evaluation of a floating-point expression during translation yields the same result as the evaluation of the same expression (or the same operations on the same values) during program execution.

[xismn]

Thanks! Didn't know about this. Unfortunately my compiler doesn't seem to support constexpr (My IDE is Visual Studio 2013).

[JLBorges]

Workaround:

namespace foo {
    
    const int ia = 2 ;
    const double a = ia / 1.0 ;
}

class Test {

    public:
    	Test() {}
    
    	const static int bar = foo::ia ;
};

http://coliru.stacked-crooked.com/a/e00a3a1dc02431e5