G705: match Excel single-character casing

crates/functions/src/text.rs

@@ -111,12 +111,27 @@ fn len(a: &[Arg], _c: &mut dyn Ctx) -> R<Value> {
     num_val(text(a, 0)?.chars().count() as f64)
 }
 
+fn lower_char(ch: char) -> char {
+    match ch.to_lowercase().next() {
+        Some(lower) => lower,
+        None => ch,
+    }
+}
+
+fn upper_char(ch: char) -> char {
+    let mut uppercase = ch.to_uppercase();
+    match uppercase.next() {
+        Some(upper) if uppercase.next().is_none() => upper,
+        _ => ch,
+    }
+}
+
 fn lower(a: &[Arg], _c: &mut dyn Ctx) -> R<Value> {
-    text_val(text(a, 0)?.to_lowercase())
+    text_val(text(a, 0)?.chars().map(lower_char).collect())
 }
 
 fn upper(a: &[Arg], _c: &mut dyn Ctx) -> R<Value> {
-    text_val(text(a, 0)?.to_uppercase())
+    text_val(text(a, 0)?.chars().map(upper_char).collect())
 }
 
 fn proper(a: &[Arg], _c: &mut dyn Ctx) -> R<Value> {
@@ -126,9 +141,9 @@ fn proper(a: &[Arg], _c: &mut dyn Ctx) -> R<Value> {
     for ch in s.chars() {
         if ch.is_alphabetic() {
             if prev_letter {
-                out.extend(ch.to_lowercase());
+                out.push(lower_char(ch));
             } else {
-                out.extend(ch.to_uppercase());
+                out.push(upper_char(ch));
             }
             prev_letter = true;
         } else {
@@ -1123,6 +1138,20 @@ mod tests {
     fn case_and_cleanup() {
         is_text(ev("UPPER", vec![t("abc")]), "ABC");
         is_text(ev("LOWER", vec![t("ABC")]), "abc");
+        is_text(ev("UPPER", vec![t("Straße")]), "STRAßE");
+        is_text(ev("LOWER", vec![t("İ")]), "i");
+        is_text(ev("PROPER", vec![t("İSTANBUL")]), "Istanbul");
+        is_text(ev("PROPER", vec![t("ß")]), "ß");
+        is_text(ev("PROPER", vec![t("ÉCOLE")]), "École");
+        let proper = ev("PROPER", vec![t("İSTANBUL Straße")]);
+        is_text(proper.clone(), "Istanbul Straße");
+        assert_eq!(
+            match proper {
+                Value::Text(s) => s.chars().count(),
+                _ => 0,
+            },
+            "Istanbul Straße".chars().count()
+        );
         is_text(ev("PROPER", vec![t("this is a TITLE")]), "This Is A Title");
         is_text(ev("PROPER", vec![t("2-way street's")]), "2-Way Street'S");
         is_text(ev("TRIM", vec![t("  a   b  c ")]), "a b c");

docs/function-parity.md

@@ -1,6 +1,6 @@
 # Function parity: worksheet functions and formulas
 
-> **Last reviewed:** 2026-10-15 · **Last updated:** 2026-10-25 · **Change:** minor (the LAMBDA row lists the recursion limit and its `#NUM!`, and the error a defined name holding an error gives when called; earlier: number text with accounting parentheses coerces like Excel 365; INDEX with one index on a 2-D reference is #REF! as in Excel; earlier: SUBTOTAL and AGGREGATE cap the cells they visit, #2175) · **Target:** Microsoft Excel (Microsoft 365), Excel for Mac 16.113.4
+> **Last reviewed:** 2026-10-31 · **Last updated:** 2026-10-31 · **Change:** minor (UPPER, LOWER and PROPER use single-character Unicode casing to match measured Excel results; locale check: Windows Excel 16.0 build 20527, LCID 1033) · **Target:** Microsoft Excel (Microsoft 365), Excel for Mac 16.113.4
 
 Excel's worksheet functions, category by category, against GridCraft's function library. Part of
 [target-app-parity.md](target-app-parity.md); gaps feed [gaps.md](gaps.md).
@@ -123,6 +123,7 @@ workbooks; results checked in as data, not Excel files) and fixing what it finds
 
 | Date | Change | Summary |
 |---|---|---|
+| 2026-10-31 | minor | UPPER, LOWER and PROPER use single-character casing: `UPPER("Straße")` keeps ß, `LOWER("İ")` returns i, and `PROPER("İSTANBUL")` returns Istanbul, matching Windows Excel 16.0 build 20527 in LCID 1033 |
 | 2026-10-25 | minor | LAMBDA row: a LAMBDA that calls itself more than 140 times, and a formula nested past the depth limit, give `#NUM!` (#1809; nesting gave `#CALC!` before). `=Foo(1)` where the defined name Foo holds `#N/A` gives `#N/A`, not `#NAME?` |
 | 2026-10-23 | minor | Number text coercion (VALUE, arithmetic, typed entry): a dollar sign before accounting parentheses (`$(1,234)`, `$ (5)`) and a percent sign after them (`(5)%`) read as negatives; `(-5)`, `((5))`, `($-5)`, `$5%`, `$(5)%` and dates or times in parentheses are `#VALUE!`, as observed in Excel 365 |
 | 2026-10-23 | minor | INDEX with only a row index on a reference of several rows and columns (`INDEX(A1:C3,2)`, `INDEX(A1:C3,0)`) is `#REF!`, as observed in Excel, so it also fails as a range endpoint and inside other functions; an empty column argument (`INDEX(A1:C3,2,)`) still gives the row. Checked that a column array with one index stays a 1×1 array (`TYPE` 64), which is what Excel returns |