M2compiler-V3/docs/summary_polish-ustr-iochan.md

Step: UString concat/LEN + IOChan position/seek

Tag v3-polish-ustr-iochan. Suite 137/137; fixpoint OK (image 2,156,095 bytes).

Two of the three remaining polish items; Clarion CLASS lowering is scoped separately (docs/summary_class-lowering.md).

UString concatenation and length

• U"a" + U"b" concatenates descriptors: QbeGen.UStrCat → shim m2ustrcat (static buffer; consume before the next concat).
• A single-codepoint operand (U"x" is a UCHAR) becomes a 1-codepoint UString via QbeGen.UStrFrom → m2ustrfrom. The shim uses a small rotating pool so an expression can supply both operands before m2ustrcat reads them.
• LEN/LENGTH/HIGH of a U"…" literal read the descriptor header: QbeGen.UStrLen → m2ustrlen (and DecQ for HIGH). A single-codepoint literal is LEN = 1, HIGH = 0.
• + handles UString+UString, UString+UCHAR, UCHAR+UString and UCHAR+UCHAR; the result type is ClUStr.

Note: LEN(expr) where the operand is a computed expression (e.g. LEN(U"a" + U"b")) is not accepted — LEN/HIGH take a designator or a literal, as before.

IOChan positioning

• Position(cid):INTEGER, Seek(cid, pos):BOOLEAN, Rewind(cid):BOOLEAN.
• Over SysShim.ftell/fseek → shim m2ftell/m2fseek (fseek(..., SEEK_SET) + clearerr on rewind).

Tests

• t_ustrcat.mod (63): LEN/LENGTH/HIGH of literals, U"ab"+U"cd", U"x"+U"é".
• iochanseek_prog.mod (15): write 6 bytes, Position, reopen, Position after reads, Rewind, Seek(3), read line back.

Files

compiler/src/{QbeGen.def,QbeGen.mod} (UStrCat/UStrFrom/ UStrLen/DecQ), compiler/src/{SymTab.def,SymTab.mod} (UStrCompat), compiler/src/M2.atg (UString + and LEN), runtime/syslib/shim.c (m2ustrcat/m2ustrfrom/m2ustrlen/ m2ftell/m2fseek), runtime/syslib/SysShim.def/.mod, stdlib/iochan.def/.mod, compiler/tests/{t_ustrcat,iochanseek_prog}.mod, compiler/run_tests.sh, docs/features.md.