Bug 15805 – Automatic fix two erroneous integer comparison cases by widening

The front end could automatically fix two cases where comparing two integers gives in wrong results. The trick is to widen the unsigned argument in a signed value, for example int a = -1; uint b; if (a > b) {} "b" can be widened in a long, which fixes the comaprison results. POC in a simple template: import std.traits; bool safeIntegralCmp(string op, L, R)(auto ref L lhs, auto ref R rhs) if (isIntegral!R && isIntegral!L) { // safe static if (is(Unqual!L == Unqual!R)) { mixin("return lhs" ~ op ~ "rhs;"); } else { // promote unsigned to bigger signed static if (isSigned!L && !isSigned!R && R.sizeof < 8) { long widenedRhs = rhs; mixin("return lhs" ~ op ~ "widenedRhs;"); } else static if (isSigned!R && !isSigned!L && L.sizeof < 8) { long widenedLhs = lhs; mixin("return widened" ~ op ~ "rhs;"); } // not fixable by operand widening else { pragma(msg, "warning, comparing a" ~ L.stringof ~ " with a" ~ R.stringof ~ " may result into wrong results"); mixin("return lhs" ~ op ~ "rhs;"); } } } unittest { int a = -1; uint b; assert(a > b); // wrong result assert(safeIntegralCmp!">"(a,b) == false); // fixed by promotion long aa = -1; ulong bb; assert(aa > bb); // wrong result assert(safeIntegralCmp!">"(aa,bb) == true); // not staticaly fixable, warning } The widening that happends in the CT branches * static if (isSigned!L && !isSigned!R && R.sizeof < 8) * static if (isSigned!R && !isSigned!L && L.sizeof < 8) could be done in the compiler.