Bug 4652 – Compiler hangs on template with zero-length tuple and another argument

The following code simply hangs DMD 2.048. I haven't the slightest clue why. struct Foo(Args...) if(Args.length > 1) { Args args; } template Foo(Arg) { alias Arg Foo; } Foo!(Args) getFoo(Args...)(Args args) { static if(Args.length > 1) { return Foo!(Args)(args); } else { return args[0]; } } Foo!(Args) getFoo(Args...)(Args args, uint num) { static if(Args.length > 1) { return Foo!(Args)(args); } else { return args[0]; } } void main() { getFoo(1); }

Bug exists in 2.020. Not a regression.

Reduced test case. ------- void bug4652(T...)(T x, int n){} void instantiate4652() { bug4652(1); }

And it fails on D1, as well (as far back as D1.028).

This is caused by a confusion in the code for deduceFunctionTemplateMatch. The comment and loop condition implies that we're looping over the function _parameters_. But the rest of the loop assumes it's a loop over the function _arguments_. In the case where the tuple is of length zero, an infinite loop results. Apart from the ICE, this also causes valid code to be rejected. ------ void bug4652(U, T...)(long y, T x, U num){} void instantiate4652() { bug4652(2, 'c', 27, 'e', 'f',1); // rejects-valid bug4652(2, 1); // infinite loop on valid code } ---- PATCH: template.c line 1090. Replace the first section of code, with the second. --------------- // Loop through the function parameters for (i = 0; i < nfparams; i++) { /* Skip over function parameters which wound up * as part of a template tuple parameter. */ if (i == fptupindex) { if (fptupindex == nfparams - 1) break; i += tuple_dim - 1; continue; } Parameter *fparam = Parameter::getNth(fparameters, i); --------------- // Loop through the function arguments for (i = 0; i < nfargs; i++) { /* Skip over function parameters which wound up * as part of a template tuple parameter. */ if (i >= fptupindex && i< fptupindex+tuple_dim) continue; /* Function parameters correspond to function arguments as follows. * Note that tuple_dim may be zero. * arg [0..fptupindex] == param[0..fptupindex] * arg [fptupindex..fptupindex+tuple_dim] == param[fptupindex] * arg[fputupindex+dim..nfargs] == param[fptupindex+1..nfparams] */ size_t parami = i; if (fptupindex >= 0 && i >= fptupindex) { parami -= tuple_dim-1; } Parameter *fparam = Parameter::getNth(fparameters, parami); --------

That patch was incorrect, it failed to deal with default and variadic parameters. This new test case incorporates the test case from bug 4676 as well, which is also fixed by this patch. --- void bug4652(U, T...)(long y, T x, U num){} void bug4652default(T) (T value, int x=2) {} void bug4652default(T) (T value, int y){ } void bug4676(T...)(T args, string str) {} void bug4676(T...)(T args) {} void instantiate4652() { bug4652(2, 'c', 27, 'e', 'f',1); // rejects-valid bug4652(2, 1); // infinite loop on valid code bug4652default(true); bug4676(1, 2, 3); } --- Revised patch. Template.c, line 1090, deduceFunctionTemplateMatch(). ========================== #endif // Loop through the function parameters - for (i = 0; i < nfparams; i++) + for (size_t parami = 0; parami < nfparams; parami++) { /* Skip over function parameters which wound up * as part of a template tuple parameter. */ - if (i == fptupindex) - { if (fptupindex == nfparams - 1) - break; + if (parami == fptupindex) + continue; + /* Set i = index into function arguments + * Function parameters correspond to function arguments as follows. + * Note that tuple_dim may be zero, and there may be default or + * variadic arguments at the end. + * arg [0..fptupindex] == param[0..fptupindex] + * arg [fptupindex..fptupindex+tuple_dim] == param[fptupindex] + * arg[fputupindex+dim.. ] == param[fptupindex+1.. ] + */ + i = parami; + if (fptupindex >= 0 && parami > fptupindex) i += tuple_dim - 1; - continue; - } - Parameter *fparam = Parameter::getNth(fparameters, i); + Parameter *fparam = Parameter::getNth(fparameters, parami); if (i >= nfargs) // if not enough arguments {