feat(codegen): convert implicitly between CHAR and STRING

A CHAR becomes a one-character STRING in &, comparisons, assignment,
parameter passing and RETURN. A STRING becomes a CHAR in assignment,
parameter passing and RETURN, with a runtime error when its length is
not 1. CHAR with INTEGER or BOOLEAN still does not convert.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

1 files changed, 10 insertions, 0 deletions

diff --git a/AGENTS/CONVENTIONS/STRINGS.md b/AGENTS/CONVENTIONS/STRINGS.md
index 2908167..9bb81cf 100644
--- a/AGENTS/CONVENTIONS/STRINGS.md
+++ b/AGENTS/CONVENTIONS/STRINGS.md
@@ -11,6 +11,16 @@ are ordinary library definitions, with exact overload selection. Comparisons use
 case-sensitive byte ordering. A missing STRING class or required bridge method is a
 compile error.
 
+**CHAR and STRING convert implicitly.** `emitCoercedExpr` turns a CHAR into a
+one-character STRING (`emitStringFromChar`, STRING.NEW(1, ch)) and a STRING into a CHAR
+(`emitCharFromString`, via the `_LENGTH()` and `C_STR()` bridge methods), so assignment,
+parameter passing and RETURN need no special cases; a length other than 1 is a runtime
+error. `resolveOperator` and `resolveCall` fall back to the STRING overload when an
+exact match is missing and a CHAR is involved (`'a' & 'b'`, `"a" = 'a'`); two CHAR
+operands still compare as CHARs, and CASE converts the CHAR side itself. A converted
+value is a fresh temporary, so `emitAssignment` copies its source (CopySource) only when
+the source type equals the target type. CHAR with INTEGER or BOOLEAN never converts.
+
 **Ownership follows value copies.** STRING constructors/assignment prevent shared
 handles. Records and BYVAL arrays containing STRING recursively copy their
 string-bearing fields/elements; RETURN and temporary cleanup preserve ownership.