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SysUtils.Pos returned 1 for an empty substring

Symptom

Pos('', 'abc')     FPC: 0     pxx: 1

C's strstr returns a hit at the start for an empty needle; Delphi and FPC both return 0 (not found). Pos had an explicit if m = 0 then Result := 1 early-out, so this was a deliberate choice made against the wrong reference.

Why it matters more than it looks

The idiomatic guard is

if Pos(sep, s) > 0 then ...

which under the old behaviour fires for an empty separator and then indexes from a position that matched nothing. It is a silent-wrong-value shape, not a crash — the caller proceeds with a plausible index.

PosEx in lib/rtl/strutils.pas already returned 0 here, so the two search functions in the RTL disagreed with each other, which is how it went unnoticed: whichever one a caller reached for looked self-consistent.

Fix

lib/rtl/sysutils.pas — return 0, matching PosEx and FPC.

Checked, not affected

The compiler builtin Pos (the no-uses intrinsic lowered to a builtin helper, covered by test/test_upcase_pos.pas) is a separate implementation and was already correct for all three empty cases — verified before assuming, since a matching bug there would have been Track A.

Test

test/lib_strutil.pas — pos-empty, pos-empty-var, pos-both-empty, plus found/absent controls. Both the literal and the variable form are asserted because a literal '' could plausibly be constant-folded down a different path. 3 fail without the fix.