具有内部链接的变量的暂定定义具有不完整的非数组类型：一致的实现显示不同的行为
Tentative definition of variable with internal linkage has incomplete non-array type: conforming implementations show different behavior
后续问题：。
上下文：具有内部链接的变量的暂定定义具有不完整的非数组类型：一致的实现显示不同的行为。
示例代码（t940.c）：
static struct s foo;
static struct s {int a;} foo;
调用：
$ gcc t940.c -c -std=c11 -pedantic -Wall -Wextra -Wno-unused-variable
<nothing>
$ clang t940.c -c -std=c11 -pedantic -Wall -Wextra -Wno-unused-variable
t940.c:1:17: warning: tentative definition of variable with internal linkage has incomplete non-array type 'struct s' [-Wtentative-definition-incomplete-type]
static struct s foo;
^
t940.c:1:15: note: forward declaration of 'struct s'
static struct s foo;
^
1 warning generated.
$ cl t940.c /c /std:c11 /Za
Microsoft (R) C/C++ Optimizing Compiler Version 19.29.30133 for x64
Copyright (C) Microsoft Corporation. All rights reserved.
t940.c
t940.c(1): error C2079: 'foo' uses undefined struct 's'
t940.c(2): error C2371: 'foo': redefinition; different basic types
t940.c(1): note: see declaration of 'foo'
$ icl t940.c -c -std=c11 -pedantic -Wno-unused-variable
<nothing>
现场演示：https://godbolt.org/z/9j8E1634q。
工具版本：
$ gcc --version
gcc (GCC) 11.2.0
$ clang --version
clang version 13.0.0
$ cl
Microsoft (R) C/C++ Optimizing Compiler Version 19.29.30133 for x64
# icl is x86-64 icc 2021.1.2 (from godbolt.org)
问题：根据 C11，哪种行为是正确的？
UPD1。创建时间：https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102821.
来自 C 标准（6.9.2 外部对象定义）
3 If the declaration of an identifier for an object is a tentative
definition and has internal linkage, the declared type shall not be an
incomplete type.
和（4.一致性）
1 In this International Standard, ‘‘shall’’ is to be interpreted as a
requirement on an implementation or on a program; conversely,
‘‘shall not’’ is to be interpreted as a prohibition.
后续问题：
上下文：具有内部链接的变量的暂定定义具有不完整的非数组类型：一致的实现显示不同的行为。
示例代码（t940.c）：
static struct s foo;
static struct s {int a;} foo;
调用：
$ gcc t940.c -c -std=c11 -pedantic -Wall -Wextra -Wno-unused-variable
<nothing>
$ clang t940.c -c -std=c11 -pedantic -Wall -Wextra -Wno-unused-variable
t940.c:1:17: warning: tentative definition of variable with internal linkage has incomplete non-array type 'struct s' [-Wtentative-definition-incomplete-type]
static struct s foo;
^
t940.c:1:15: note: forward declaration of 'struct s'
static struct s foo;
^
1 warning generated.
$ cl t940.c /c /std:c11 /Za
Microsoft (R) C/C++ Optimizing Compiler Version 19.29.30133 for x64
Copyright (C) Microsoft Corporation. All rights reserved.
t940.c
t940.c(1): error C2079: 'foo' uses undefined struct 's'
t940.c(2): error C2371: 'foo': redefinition; different basic types
t940.c(1): note: see declaration of 'foo'
$ icl t940.c -c -std=c11 -pedantic -Wno-unused-variable
<nothing>
现场演示：https://godbolt.org/z/9j8E1634q。
工具版本：
$ gcc --version
gcc (GCC) 11.2.0
$ clang --version
clang version 13.0.0
$ cl
Microsoft (R) C/C++ Optimizing Compiler Version 19.29.30133 for x64
# icl is x86-64 icc 2021.1.2 (from godbolt.org)
问题：根据 C11，哪种行为是正确的？
UPD1。创建时间：https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102821.
来自 C 标准（6.9.2 外部对象定义）
3 If the declaration of an identifier for an object is a tentative definition and has internal linkage, the declared type shall not be an incomplete type.
和（4.一致性）
1 In this International Standard, ‘‘shall’’ is to be interpreted as a requirement on an implementation or on a program; conversely, ‘‘shall not’’ is to be interpreted as a prohibition.