Merge PR #2994

# Conflicts:
#	docs/function-parity.md

crates/functions/src/text.rs

@@ -111,12 +111,31 @@ 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 proper_upper_char(ch: char) -> char {
+    if ch == '\u{130}' { 'I' } else { upper_char(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::<String>())
 }
 
 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::<String>())
 }
 
 fn proper(a: &[Arg], _c: &mut dyn Ctx) -> R<Value> {
@@ -126,9 +145,9 @@ 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(proper_upper_char(ch));
             }
             prev_letter = true;
         } else {
@@ -1127,6 +1146,20 @@ 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:** 2026-10-31 · **Last updated:** 2026-10-31 · **Change:** minor (UNICHAR returns `#N/A` for Unicode noncharacters, as observed in Excel; earlier: BITLSHIFT and BITRSHIFT return zero for zero operands with shifts beyond 53 bits, as observed in Excel; earlier: ECMA.CEILING listed with USDOLLAR as not on Microsoft's list; IMSQRT reproduces Excel's stray real part for negative reals) · **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; earlier: UNICHAR returns `#N/A` for Unicode noncharacters, as observed in Excel; earlier: ECMA.CEILING listed with USDOLLAR as not on Microsoft's list; IMSQRT reproduces the stray real part Excel gives for negative reals, and the other IM* differences from Excel are listed; the LAMBDA row lists the recursion limit and its `#NUM!`, and the error a defined name holding an error gives when called; number text with accounting parentheses coerces like Excel 365; INDEX with one index on a 2-D reference is #REF! as in Excel; SUBTOTAL and AGGREGATE cap the cells they visit, #2175) · **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).
@@ -128,6 +128,7 @@ workbooks; results checked in as data, not Excel files) and fixing what it finds
 | 2026-10-31 | minor | DEC2BIN, DEC2HEX and DEC2OCT validate `places` before returning negative two's-complement output, and Boolean places return `#VALUE!`, matching Excel |
 | 2026-10-31 | minor | BITLSHIFT and BITRSHIFT return `0` for zero operands shifted beyond 53 bits in either direction; nonzero operands retain the `#NUM!` limit, matching Excel |
 | 2026-10-31 | minor | UNICHAR returns `#N/A` for U+FDD0–U+FDEF and U+FFFE/U+FFFF on every plane; adjacent valid characters and existing invalid-range/surrogate errors remain unchanged, matching Excel |
+| 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 |
 | 2026-10-27 | minor | ECMA.CEILING added to the functions that are in our library but not on Microsoft's list (with USDOLLAR), so the counts don't change |
 | 2026-10-26 | minor | IMSQRT uses Excel's polar route with its cosine reduction, so `IMSQRT("-4")` is `1.22514845490862E-16+2i` as in Excel (exact `2i` before); the rows list the IM* differences still open |
 | 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?` |