[llvm-bugs] [Bug 46255] New: Anonymous structs/unions can violate "Last member element" constraint on flexible arrays

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=46255

            Bug ID: 46255
           Summary: Anonymous structs/unions can violate "Last member
                    element" constraint on flexible arrays
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: C
          Assignee: unassignedclangbugs at nondot.org
          Reporter: vaibhav at linux.ibm.com
                CC: blitzrakete at gmail.com, dgregor at apple.com,
                    erik.pilkington at gmail.com, llvm-bugs at lists.llvm.org,
                    richard-llvm at metafoo.co.uk

Description:

In 'C' mode defining a struct containing 2 or more structs having flexible
arrays should result in 'gnu-variable-sized-type-not-at-end' warning getting
triggered as flexible arrays should always be at the end of the containing
struct.

However if the member struct elements are wrapped in an anonymous union or a
struct then clang erroneously ignores this violation.

Example:
Consider following struct definitions ( https://godbolt.org/z/7nnBGz ):

struct A { int a; int b[]; };
struct B { int a; int b[]; };

struct C {
/* warning: field 'a' with variable sized type 'struct A' not at
 * the end of a struct or class is a GNU extension 
 * [-Werror,-Wgnu-variable-sized-type-not-at-end]
 */ 
    struct A a;  
    struct B b;
};

struct C_U { 
     /* No warning reported */
    struct A a;
    union {
          struct B b;  
    };
};

struct C_S { 
    /* No warning reported */
    struct A a;
    struct {
          struct B b;  
    };
};

struct C_SU { 
    /* No warning reported */
    struct A a;
    struct {
          struct B b;  
    };
    union {
          struct A aa;  
    };
};


Exception:
This issue is not observed in 'C++' mode ( https://godbolt.org/z/3nNzap )

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