Lookup of operator[] succeeds when should be ambiguous

[llvmbot]

Bugzilla Link	26850
Version	unspecified
OS	Windows NT
Reporter	LLVM Bugzilla Contributor
CC	@riccibruno

Extended Description

Consider the following reduced code example from a StackOverflow question:

struct X{};
struct Y{};

struct B
{
    void operator[](X){}
};

struct C
{
    void operator[](Y){}
};

struct D : B, C {};

int main()
{
    D d;
    d[Y()];
}

The call to D::operator[] should be ambiguous due to the two different base class overloads. A call to d.operator[](Y()) would correctly fail to compile.

[zygoloid]

Duplicate of #22413.

[llvmbot]

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