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Lingxi
Lingxi

Reputation: 14977

Issues concerning const decltype(x)&

Consider the following code:

int a = 1;
const int& b = a;
std::cout << std::is_same<const decltype(b)&, const int&>();

It compiles on clang 3.5 while GCC 4.9 gives the following error:

error: 'const' qualifiers cannot be applied to 'const int&'

Which one is correct according to the standard? My guess is that GCC is standard-conformant, just as you can't do int& const b = a;.

Upvotes: 9

Views: 123

Answers (1)

Kerrek SB
Kerrek SB

Reputation: 477160

I believe the code is valid and the two types are the same.

[dcl.ref]/1 says:

Cv-qualified references are ill-formed except when the cv-qualifiers are introduced through the use of a typedef-name (7.1.3, 14.1) or decltype-specifier (7.1.6.2), in which case the cv-qualifiers are ignored.

Since you are introducing the first const through a decltype-specifier, it is ignored, and your first type is equivalent to decltype(b)&.

Now [dcl.ref]/6 says:

If a typedef-name (7.1.3, 14.1) or a decltype-specifier (7.1.6.2) denotes a type TR that is a reference to a type T, an attempt to create the type "lvalue reference to cv TR" creates the type "lvalue reference to T" [...]

Your decltype-specifier denotes the type "reference to const int", and you are attempting to create an lvalue reference, so you end up with an lvalue reference to const int.

Upvotes: 3

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