unicode: fix SpecialCase to follow its docs & respect explict no-op mappings

If SpecialCase contains an explicit CaseRange with zero deltas,
respect those and don't fall back to the default behavior.

Fixes #25636

Change-Id: Ic554c6b3dd462b1b39c75194eec469b6ff4aa55b
Reviewed-on: https://go-review.googlesource.com/117155
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>

src/unicode/letter.go

@@ -206,9 +206,10 @@ func IsTitle(r rune) bool {
 }
 
 // to maps the rune using the specified case mapping.
-func to(_case int, r rune, caseRange []CaseRange) rune {
+// It additionally reports whether caseRange contained a mapping for r.
+func to(_case int, r rune, caseRange []CaseRange) (mappedRune rune, foundMapping bool) {
 	if _case < 0 || MaxCase <= _case {
-		return ReplacementChar // as reasonable an error as any
+		return ReplacementChar, false // as reasonable an error as any
 	}
 	// binary search over ranges
 	lo := 0
@@ -229,9 +230,9 @@ func to(_case int, r rune, caseRange []CaseRange) rune {
 				// bit in the sequence offset.
 				// The constants UpperCase and TitleCase are even while LowerCase
 				// is odd so we take the low bit from _case.
-				return rune(cr.Lo) + ((r-rune(cr.Lo))&^1 | rune(_case&1))
+				return rune(cr.Lo) + ((r-rune(cr.Lo))&^1 | rune(_case&1)), true
 			}
-			return r + delta
+			return r + delta, true
 		}
 		if r < rune(cr.Lo) {
 			hi = m
@@ -239,12 +240,13 @@ func to(_case int, r rune, caseRange []CaseRange) rune {
 			lo = m + 1
 		}
 	}
-	return r
+	return r, false
 }
 
 // To maps the rune to the specified case: UpperCase, LowerCase, or TitleCase.
 func To(_case int, r rune) rune {
-	return to(_case, r, CaseRanges)
+	r, _ = to(_case, r, CaseRanges)
+	return r
 }
 
 // ToUpper maps the rune to upper case.
@@ -282,8 +284,8 @@ func ToTitle(r rune) rune {
 
 // ToUpper maps the rune to upper case giving priority to the special mapping.
 func (special SpecialCase) ToUpper(r rune) rune {
-	r1 := to(UpperCase, r, []CaseRange(special))
-	if r1 == r {
+	r1, hadMapping := to(UpperCase, r, []CaseRange(special))
+	if r1 == r && !hadMapping {
 		r1 = ToUpper(r)
 	}
 	return r1
@@ -291,8 +293,8 @@ func (special SpecialCase) ToUpper(r rune) rune {
 
 // ToTitle maps the rune to title case giving priority to the special mapping.
 func (special SpecialCase) ToTitle(r rune) rune {
-	r1 := to(TitleCase, r, []CaseRange(special))
-	if r1 == r {
+	r1, hadMapping := to(TitleCase, r, []CaseRange(special))
+	if r1 == r && !hadMapping {
 		r1 = ToTitle(r)
 	}
 	return r1
@@ -300,8 +302,8 @@ func (special SpecialCase) ToTitle(r rune) rune {
 
 // ToLower maps the rune to lower case giving priority to the special mapping.
 func (special SpecialCase) ToLower(r rune) rune {
-	r1 := to(LowerCase, r, []CaseRange(special))
-	if r1 == r {
+	r1, hadMapping := to(LowerCase, r, []CaseRange(special))
+	if r1 == r && !hadMapping {
 		r1 = ToLower(r)
 	}
 	return r1

src/unicode/letter_test.go

@@ -9,6 +9,7 @@ import (
 	"fmt"
 	"runtime"
 	"sort"
+	"strings"
 	"testing"
 	. "unicode"
 )
@@ -551,3 +552,14 @@ func TestLatinOffset(t *testing.T) {
 		}
 	}
 }
+
+func TestSpecialCaseNoMapping(t *testing.T) {
+	// Issue 25636
+	// no change for rune 'A', zero delta, under upper/lower/title case change.
+	var noChangeForCapitalA = CaseRange{'A', 'A', [MaxCase]rune{0, 0, 0}}
+	got := strings.ToLowerSpecial(SpecialCase([]CaseRange{noChangeForCapitalA}), "ABC")
+	want := "Abc"
+	if got != want {
+		t.Errorf("got %q; want %q", got, want)
+	}
+}