getting: "error: initializer element is not a compile-time constant" Even when struct is inside function

[aalmkainzi]

int foo(int *a, int b)
{
    return 0;
}

int main()
{
    int x;
    (struct {
          typeof(foo(&(struct { int t; }){.t = x}.t, 0)) t;
        }){0};
}

the above code compiles with gcc and tcc. but fails with clang.

The error I get is:

err_test.c:10:48: error: initializer element is not a compile-time constant
          typeof(foo(&(struct { int t; }){.t = x}.t, 0)) t;
                                               ^
1 error generated.

This error seems to be for global or static variables, but for some reason is triggering even though the variable is defined inside a function and inside a typeof

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Abdulmalek Almkainzi (aalmkainzi)

```C int foo(int *a, int b) { return 0; }

int main()
{
int x;
(struct {
typeof(foo(&(struct { int t; }){.t = x}.t, 0)) t;
}){0};
}

the above code compiles with gcc and tcc. but fails with clang.

The error I get is: 

err_test.c:10:48: error: initializer element is not a compile-time constant
typeof(foo(&(struct { int t; }){.t = x}.t, 0)) t;
^
1 error generated.

This error seems to be for global or static variables, but for some reason is triggering even though the variable is defined inside a function and inside a `typeof`
</details>

[shafik]

x does not look like a constant expression in C, I think gcc is accepting that as an implementation defined constant expression.

However if we change x to be const or constexpr and initialize it then both accept it: https://godbolt.org/z/jT6Yzoje3

This makes a lot more sense, although the const case feels like an extension.

CC @AaronBallman @cor3ntin @Fznamznon

[aalmkainzi]

x does not look like a constant expression in C, I think gcc is accepting that as an implementation defined constant expression.

However if we change x to be const or constexpr and initialize it then both accept it: https://godbolt.org/z/jT6Yzoje3

This makes a lot more sense, although the const case feels like an extension.

CC @AaronBallman @cor3ntin @Fznamznon

Why would x be required to be constant in this context?

[AaronBallman]

It's not required to be a constant in this case. The compound literal is at block scope and so the initializer does not need to be constant (C23 6.5.3.6p4).

The issue is that we're checking for file scope at

llvm-project/clang/lib/Sema/SemaExpr.cpp

Line 7863 in f6357bb

bool isFileScope = !CurContext->isFunctionOrMethod();

and because the context is "record scope" we assume it must be file scope.

[aalmkainzi]

shouldn't also work for file scope? since the compound literal is inside a typeof

[AaronBallman]

shouldn't also work for file scope? since the compound literal is inside a typeof

There are no special allowances for initialization that occurs in an unevaluated context, so being inside of typeof (or sizeof, etc) makes no difference.