[llvm-bugs] [Bug 35028] New: Clang does not use the strictest alignas in templates

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=35028

            Bug ID: 35028
           Summary: Clang does not use the strictest alignas in templates
           Product: clang
           Version: 5.0
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: billy.oneal at gmail.com
                CC: dgregor at apple.com, llvm-bugs at lists.llvm.org

template<class X>
struct alignas(X) alignas(unsigned long) alignas(unsigned int) queue {
    unsigned long a;
    unsigned int b;
    // X suffix[];
};

template struct queue<unsigned short>;

On Godbolt: https://godbolt.org/g/7MUaSE

2 : <source>:2:8: error: requested alignment is less than minimum alignment of
8 for type 'queue<unsigned short>'
struct alignas(X) alignas(unsigned long) alignas(unsigned int) queue {
       ^
8 : <source>:8:17: note: in instantiation of template class 'queue<unsigned
short>' requested here
template struct queue<unsigned short>;
                ^
1 error generated.
Compiler exited with result code

N4687 10.6.2 [dcl.align]/4 says:
The alignment requirement of an entity is the strictest nonzero alignment
specified by its alignment-specifiers, if any; otherwise, the
alignment-specifiers have no effect.

Therefore, even though alignas(X) requests an alignment of 2, less than the
minimum of 8, the following alignas which requests an alignment of 8 should
make the overall alignment request be for 8.

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