[llvm-bugs] [Bug 38234] New: Partial specialization halfway accepted after instantiation

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=38234

            Bug ID: 38234
           Summary: Partial specialization halfway accepted after
                    instantiation
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: zhonghao at pku.org.cn
                CC: dgregor at apple.com, llvm-bugs at lists.llvm.org

The following code produces predictable but odd output. The compiler should
either generate a warning for this case or throw away the instantiation of the
type upon encountering the better-matching partial specialization, somehow
keeping it for code above the given line.


template <typename T>
struct A {
        T o;
};

int sizeofa = sizeof(A<int *>);

template <typename T>
struct A<T *> {
        T o[5];
};

int newsizeofa = sizeof(A<int *>);

int main() {
        printf("%d, %d, %d\n", sizeofa, sizeof(A<int *>), sizeof(A<double *>));
}


The given output is 8, 8, 40. It appears that the specialization is accepted
for any type not yet instantiated.

I tried g++. It produces an error message:

code1.cpp:12:8: error: partial specialization of 'struct A<T*>' after
instantiation of 'struct A<int*>' [-fpermissive]
 struct A<T *> {
        ^~~~~~

A simpler testcase:

template <class T> struct A { };
A<int*> a;
template <class T> struct A<T*> { }; // { dg-error "A<int *>" }

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