Another possible approach would be to change YYLLOC_DEFAULT to something like:

#define YYLLOC_DEFAULT(Cur, Rhs, N)                                     \
    ((Cur) = (N) ? YYRHSLOC(Rhs, 1) + YYRHSLOC(Rhs, N) : yylloc)

This would set the location to the location of the lookahead token for an epsilon reduction.

Perhaps better:

#define YYLLOC_DEFAULT(Cur, Rhs, N)                                             \
    ((Cur) = (N) ? YYRHSLOC(Rhs, 1) + YYRHSLOC(Rhs, N)                          \
        : yylloc ? yyloc \
                 : Util::SourceInfo(driver.sources, driver.sources.getSourcePosition()))

or

#define YYLLOC_DEFAULT(Cur, Rhs, N)                                             \
    ((Cur) = (N) ? YYRHSLOC(Rhs, 1) + YYRHSLOC(Rhs, N)                          \
        : YYRHSLOC(Rhs, 0) ?  Util::SourceInfo(driver.sources, YYRHSLOC(Rhs, 0).getEnd())  \
                 : Util::SourceInfo(driver.sources, driver.sources.getSourcePosition()))

Perhaps better:

#define YYLLOC_DEFAULT(Cur, Rhs, N)                                             \
    ((Cur) = (N) ? YYRHSLOC(Rhs, 1) + YYRHSLOC(Rhs, N)                          \
        : yylloc ? yyloc \
                 : Util::SourceInfo(driver.sources, driver.sources.getSourcePosition()))

or

#define YYLLOC_DEFAULT(Cur, Rhs, N)                                             \
    ((Cur) = (N) ? YYRHSLOC(Rhs, 1) + YYRHSLOC(Rhs, N)                          \
        : YYRHSLOC(Rhs, 0) ?  Util::SourceInfo(driver.sources, YYRHSLOC(Rhs, 0).getEnd())  \
                 : Util::SourceInfo(driver.sources, driver.sources.getSourcePosition()))

It might actually be nicer to have the less complex version, I looked at the reference file failures, and they seemed to be benign. Mostly a difference of 1 character in terms of start/end.

Simplest of all would be

#define YYLLOC_DEFAULT(Cur, Rhs, N)       \
    ((Cur) = (N) ? YYRHSLOC(Rhs, 1) + YYRHSLOC(Rhs, N) : YYRHSLOC(Rhs, 0))

which might be good enough. Would just sometimes make rules pull in an extra token if they involve an epsilon reduction.

or

#define YYLLOC_DEFAULT(Cur, Rhs, N)       \
    ((Cur) = (N) ? YYRHSLOC(Rhs, 1) + YYRHSLOC(Rhs, N) : Util::SrcInfo())

which would make all epsilons have an invalid srcInfo. But then the simple add would lose tokens for any rule with 3+ symbols when the first or last was an epsilon -- would be better to sum YYRHSLOC(Rhs, 1..N)

Another possibility would be

#define YYLLOC_DEFAULT(Cur, Rhs, N) Cur = yylloc_default([](int i) { return YYRHSLOC(Rhs, i); }, N)

in all the .y/.ypp files and have a single

template <typename RHS> Util::SourceInfo yylloc_default(RHS rhs, int n) {
    Util::SourceInfo rv;
    for (int i = 1; i <= n; ++) rv += rhs(i);
    return rv;
}

in a header file to avoid the complex macro with line continuations

Ended up reverting to the more elaborate version since there were small differences in the line count that led to failing tests.