Confusing diagnostics: error: 'extern' variable cannot have an initializer

[pmor13]

This C code:

static int x;

void f(void)
{
    extern int x = 1;
}

leads to:

error: 'extern' variable cannot have an initializer

which is confusing, because extern variable CAN have an initializer, which is demonstrated by the C standard (C11, 6.9.2 External object definitions, 4, EXAMPLE 1):

extern int i3 = 3; // definition, external linkage

The expected diagnostics is:

error: declaration of block scope identifier x with internal linkage shall have no initializer

Relevant quote from C11:

If the declaration of an identifier has block scope, and the identifier has external or internal linkage, the declaration shall have no initializer for the identifier.

[llvmbot]

@llvm/issue-subscribers-good-first-issue

[AaronBallman]

C2x 6.7.10p6 and C++2b [dcl.init.general]p5 both agree on the behavior, so I agree that the diagnostic can be reworded in this case. There's only one use of err_block_extern_cant_init and it is for this specific situation being diagnosed.

[pmor13]

@jakevossen5 @junaire Any interest in fixing this issue?

[junaire]

Here's a potential fix: https://reviews.llvm.org/D133088
Sorry @pmor13 I should also add you as a reviewer, but I don't know your phab name.

[pmor13]

@junaire My phab name is pmor13.

Note that this code:

int x;

void f(void)
{
    extern int x = 1;
}

needs to produce:

error" declaration of block scope identifier with external linkage shall have no initializer

So, err_block_extern_cant_init needs to be updated too (I think).