unicode: correctly handle negative runes

Is and isExcludingLatin did not handle negative runes when dispatching
to is16. TestNegativeRune covers this along with the existing uint32
casts in IsGraphic, etc. (For tests, I picked the smallest non-Latin-1
code point in each range.)

Updates #43254

Change-Id: I17261b91f0d2b5b5125d19219411b45c480df74f
Reviewed-on: https://go-review.googlesource.com/c/go/+/280493
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
Trust: Emmanuel Odeke <emmanuel@orijtech.com>

src/unicode/letter.go

@@ -154,7 +154,8 @@ func is32(ranges []Range32, r uint32) bool {
 // Is reports whether the rune is in the specified table of ranges.
 func Is(rangeTab *RangeTable, r rune) bool {
 	r16 := rangeTab.R16
-	if len(r16) > 0 && r <= rune(r16[len(r16)-1].Hi) {
+	// Compare as uint32 to correctly handle negative runes.
+	if len(r16) > 0 && uint32(r) <= uint32(r16[len(r16)-1].Hi) {
 		return is16(r16, uint16(r))
 	}
 	r32 := rangeTab.R32
@@ -166,7 +167,8 @@ func Is(rangeTab *RangeTable, r rune) bool {
 
 func isExcludingLatin(rangeTab *RangeTable, r rune) bool {
 	r16 := rangeTab.R16
-	if off := rangeTab.LatinOffset; len(r16) > off && r <= rune(r16[len(r16)-1].Hi) {
+	// Compare as uint32 to correctly handle negative runes.
+	if off := rangeTab.LatinOffset; len(r16) > off && uint32(r) <= uint32(r16[len(r16)-1].Hi) {
 		return is16(r16[off:], uint16(r))
 	}
 	r32 := rangeTab.R32

src/unicode/letter_test.go

@@ -563,3 +563,82 @@ func TestSpecialCaseNoMapping(t *testing.T) {
 		t.Errorf("got %q; want %q", got, want)
 	}
 }
+
+func TestNegativeRune(t *testing.T) {
+	// Issue 43254
+	// These tests cover negative rune handling by testing values which,
+	// when cast to uint8 or uint16, look like a particular valid rune.
+	// This package has Latin-1-specific optimizations, so we test all of
+	// Latin-1 and representative non-Latin-1 values in the character
+	// categories covered by IsGraphic, etc.
+	nonLatin1 := []uint32{
+		// Lu: LATIN CAPITAL LETTER A WITH MACRON
+		0x0100,
+		// Ll: LATIN SMALL LETTER A WITH MACRON
+		0x0101,
+		// Lt: LATIN CAPITAL LETTER D WITH SMALL LETTER Z WITH CARON
+		0x01C5,
+		// M: COMBINING GRAVE ACCENT
+		0x0300,
+		// Nd: ARABIC-INDIC DIGIT ZERO
+		0x0660,
+		// P: GREEK QUESTION MARK
+		0x037E,
+		// S: MODIFIER LETTER LEFT ARROWHEAD
+		0x02C2,
+		// Z: OGHAM SPACE MARK
+		0x1680,
+	}
+	for i := 0; i < MaxLatin1+len(nonLatin1); i++ {
+		base := uint32(i)
+		if i >= MaxLatin1 {
+			base = nonLatin1[i-MaxLatin1]
+		}
+
+		// Note r is negative, but uint8(r) == uint8(base) and
+		// uint16(r) == uint16(base).
+		r := rune(base - 1<<31)
+		if Is(Letter, r) {
+			t.Errorf("Is(Letter, 0x%x - 1<<31) = true, want false", base)
+		}
+		if IsControl(r) {
+			t.Errorf("IsControl(0x%x - 1<<31) = true, want false", base)
+		}
+		if IsDigit(r) {
+			t.Errorf("IsDigit(0x%x - 1<<31) = true, want false", base)
+		}
+		if IsGraphic(r) {
+			t.Errorf("IsGraphic(0x%x - 1<<31) = true, want false", base)
+		}
+		if IsLetter(r) {
+			t.Errorf("IsLetter(0x%x - 1<<31) = true, want false", base)
+		}
+		if IsLower(r) {
+			t.Errorf("IsLower(0x%x - 1<<31) = true, want false", base)
+		}
+		if IsMark(r) {
+			t.Errorf("IsMark(0x%x - 1<<31) = true, want false", base)
+		}
+		if IsNumber(r) {
+			t.Errorf("IsNumber(0x%x - 1<<31) = true, want false", base)
+		}
+		if IsPrint(r) {
+			t.Errorf("IsPrint(0x%x - 1<<31) = true, want false", base)
+		}
+		if IsPunct(r) {
+			t.Errorf("IsPunct(0x%x - 1<<31) = true, want false", base)
+		}
+		if IsSpace(r) {
+			t.Errorf("IsSpace(0x%x - 1<<31) = true, want false", base)
+		}
+		if IsSymbol(r) {
+			t.Errorf("IsSymbol(0x%x - 1<<31) = true, want false", base)
+		}
+		if IsTitle(r) {
+			t.Errorf("IsTitle(0x%x - 1<<31) = true, want false", base)
+		}
+		if IsUpper(r) {
+			t.Errorf("IsUpper(0x%x - 1<<31) = true, want false", base)
+		}
+	}
+}