`String(p)` on a PChar is rejected while `AnsiString(p)` works

Track P (compiler/parser.inc, the tkString_T factor).
Found 2026-08-20 by an FPC differential probe over pointers and PChar.

The measurement

fpc -O- -Mobjfpc 3.2.2 vs pxx at fe8a34230, with s := 'hello' and p := PChar(s):

expression	FPC	pxx
`t := String(p)`	hello	error: String(): operand must be Char or string
`t := String(p + 2)`	llo	same error
`t := String(PChar(@buf[0]))`	abc	same error
`t := AnsiString(p)`	hello	hello
`t := p`	hello	hello
`t := StrPas(p)`	hello	hello

One conversion, four spellings, and the broken one is the spelling real FPC code writes.

Root cause

String is a keyword (tkString_T) with its own factor branch, which handled a string operand (passthrough), a Char operand (AN_STR_FROM_CHAR) and a Variant operand (VariantCastToTemp), then errored. AnsiString is an ordinary identifier and goes through the generic built-in-typecast path, which builds an AN_PTR_CAST and works for any operand including a pointer — PChar is tyPointer here.

So the two spellings of one cast were served by two entirely separate pieces of code, and only one of them knew about pointers. That is the shape devdocs/dev/normalise-dont-special-case.md describes, and the keyword/identifier split is exactly why nobody noticed: the paths do not look like duplicates.

The fix gives the tkString_T branch the pointer arm, building the same AN_PTR_CAST the identifier path builds, with the same managed-vs-frozen choice of string kind (PXX_MANAGED_STRING).

Test

test/test_string_of_pchar.pas, 17 FPC-verified rows: String(p), String(p + 2), String(PChar(@buf)), assigned / concatenated / measured / upper-cased; an empty C string; all four spellings compared against each other in one row; and the operands String() already accepted (string, Char, Char literal, Variant) so the fix is proved not to have moved them. The pinned binary refuses the test outright.

Also found by the same probe, and fixed alongside

bug-p-inc-of-a-typed-pointer-steps-one-byte — it only became visible once this compile error was gone.

Gate