Internal compiler error - Templated conversion operator in switch expression

Question

The following code crashes the Microsoft compiler:

class Var
{
public:
    template <typename T>
    operator T () const
    { }
};

int main()
{
    Var v;
    switch (v)
    { }
}

My question: Is the code correct or should the compiler give an appropriate error? Is an unambiguous conversion to an integral type possible?

Can you be more specific when you say "crashes the compiler?" What do you mean by "is the code correct?" — Robert Harvey, Jul 30 '14 at 20:35
@RobertHarvey `fatal error C1001: An internal error has occurred in the compiler.` ... blahblah ... please report to microsoft (I did report the bug). On the line of the conversion operator. — typ1232, Jul 30 '14 at 20:36
Even if the code is incorrect, it shouldn't crash the compiler...Your `operator T` doesn't actually return anything, though, so that's UB right there even if it compiled. — T.C., Jul 30 '14 at 20:37
First of `operator T()` must return something. Second, `switch` waits for an `int`. — 101010, Jul 30 '14 at 20:38
You can only use `class Var` in `switch` statement if it can be converted into an integral value. — Thomas Matthews, Jul 30 '14 at 20:41
@40two: worse - `switch` wants "integral type, enumeration type, or of a class type for which a single non-explicit conversion function to integral or enumeration type exists" (**6.4.2/2**). If it wanted `int` specifically, it could have gotten one with `v.operator int()`. But `v` is convertible to anything, so the statement is ambiguous. — Igor Tandetnik, Jul 30 '14 at 20:42
@40two It does not matter what is inside the function body of the operator, the compiler crashes regardless. I just stripped it to keep the code minimal. — typ1232, Jul 30 '14 at 20:42
@IgorTandetnik Your comment has answer quality! Thank you for looking up the quote. — typ1232, Jul 30 '14 at 20:44
Compiler crashing (ICE) is, of course, **always a bug**. If the code is incorrect (as here), then this is merely a minor nuisance. But if the code is correct, an ICE is really annoying and requires an (often tedious) work around. — Walter, Jul 30 '14 at 20:46
@Slava I meant requires expression of integer type but felt `int` was shorter :). Sorry for unintended misleading. — 101010, Jul 30 '14 at 20:46
I tried it in Studio 2013 and also got the crash. The interesting thing is that the editor flags the `switch(v)` with a red squiggle and the pop-up "Error: expression must have integral or enum type". — Rob K, Jul 30 '14 at 20:59
@RobK Curiously, IntelliSense in VS is backed by a [compiler other than MSVC](http://blogs.msdn.com/b/vcblog/archive/2009/05/27/rebuilding-intellisense.aspx). It's common for the two to flag different sets of errors. — Igor Tandetnik, Jul 30 '14 at 21:05

score 8 · Accepted Answer · edited May 23 '17 at 12:20

The compiler crashing is always a bug, this code does not compile on either gcc or clang but both provide an error without crashing. For clang the error is:

error: statement requires expression of integer type ('Var' invalid)
switch (v)
^       ~

gcc provides the following error:

error: ambiguous default type conversion from 'Var'
 switch (v)
          ^

Also, note that flowing off the end of a value returning function is undefined behavior in C++.

Update

Adding:

operator int () const
{ return 0; }

to the class brings about different results from clang and gcc.

See Classes with both template and non-template conversion operators in the condition of switch statement for a discussion on whether gcc or clang is correct. My interpretation of N3323 implies clang is correct on this one.

Filed bug report

I filed a bug report for this ICE, so far no response. Even though this seems like an odd corner case it does cause an internal compiler error which should be fixed.

Internal compiler error - Templated conversion operator in switch expression

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