expression: cover latin1_swedish_ci in collation aggregation tests

Pin how latin1_swedish_ci behaves in inferCollation, which the new collator
reaches through isBinCollation. At equal coercibility the _bin collation wins
regardless of argument order, an explicit COLLATE beats it either way round,
two conflicting explicit collations cannot be aggregated, and both binary and
utf8mb4 take precedence over latin1 at equal coercibility.

Also extend TestCompareString with the latin1 collations, including the entries
where latin1_swedish_ci deliberately differs from the utf8 collations already
covered there: sharp s (0xDF) folds to nothing, and A-ring, A-diaeresis and
O-diaeresis sort after Z rather than onto A and O.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TgAxo4cK4qCpuLrhroHWy1

Author: terry1purcell

pkg/expression/collation_test.go

@@ -177,6 +177,85 @@ func TestInferCollation(t *testing.T) {
 			false,
 			&ExprCollation{CoercibilityImplicit, UNICODE, charset.CharsetBin, charset.CollationBin},
 		},
+		// latin1_bin mixed with latin1_swedish_ci. At equal coercibility the _bin
+		// collation wins regardless of argument order, so latin1_swedish_ci must be
+		// recognized as the non-bin side (see isBinCollation).
+		{
+			[]Expression{
+				newExpression(CoercibilityImplicit, UNICODE, charset.CharsetLatin1, charset.CollationLatin1),
+				newExpression(CoercibilityImplicit, UNICODE, charset.CharsetLatin1, "latin1_swedish_ci"),
+			},
+			false,
+			&ExprCollation{CoercibilityImplicit, UNICODE, charset.CharsetLatin1, charset.CollationLatin1},
+		},
+		{
+			[]Expression{
+				newExpression(CoercibilityImplicit, UNICODE, charset.CharsetLatin1, "latin1_swedish_ci"),
+				newExpression(CoercibilityImplicit, UNICODE, charset.CharsetLatin1, charset.CollationLatin1),
+			},
+			false,
+			&ExprCollation{CoercibilityImplicit, UNICODE, charset.CharsetLatin1, charset.CollationLatin1},
+		},
+		{
+			[]Expression{
+				newExpression(CoercibilityImplicit, UNICODE, charset.CharsetLatin1, "latin1_swedish_ci"),
+				newExpression(CoercibilityImplicit, UNICODE, charset.CharsetLatin1, "latin1_swedish_ci"),
+			},
+			false,
+			&ExprCollation{CoercibilityImplicit, UNICODE, charset.CharsetLatin1, "latin1_swedish_ci"},
+		},
+		// An explicit COLLATE still beats the _bin collation, either way round.
+		{
+			[]Expression{
+				newExpression(CoercibilityExplicit, UNICODE, charset.CharsetLatin1, "latin1_swedish_ci"),
+				newExpression(CoercibilityImplicit, UNICODE, charset.CharsetLatin1, charset.CollationLatin1),
+			},
+			false,
+			&ExprCollation{CoercibilityExplicit, UNICODE, charset.CharsetLatin1, "latin1_swedish_ci"},
+		},
+		{
+			[]Expression{
+				newExpression(CoercibilityImplicit, UNICODE, charset.CharsetLatin1, charset.CollationLatin1),
+				newExpression(CoercibilityExplicit, UNICODE, charset.CharsetLatin1, "latin1_swedish_ci"),
+			},
+			false,
+			&ExprCollation{CoercibilityExplicit, UNICODE, charset.CharsetLatin1, "latin1_swedish_ci"},
+		},
+		// Two conflicting explicit collations cannot be aggregated.
+		{
+			[]Expression{
+				newExpression(CoercibilityExplicit, UNICODE, charset.CharsetLatin1, charset.CollationLatin1),
+				newExpression(CoercibilityExplicit, UNICODE, charset.CharsetLatin1, "latin1_swedish_ci"),
+			},
+			true,
+			nil,
+		},
+		// latin1 is not a Unicode charset, so utf8mb4 wins at equal coercibility.
+		{
+			[]Expression{
+				newExpression(CoercibilityImplicit, UNICODE, charset.CharsetLatin1, "latin1_swedish_ci"),
+				newExpression(CoercibilityImplicit, UNICODE, charset.CharsetUTF8MB4, charset.CollationUTF8MB4),
+			},
+			false,
+			&ExprCollation{CoercibilityImplicit, UNICODE, charset.CharsetUTF8MB4, charset.CollationUTF8MB4},
+		},
+		{
+			[]Expression{
+				newExpression(CoercibilityImplicit, UNICODE, charset.CharsetUTF8MB4, charset.CollationUTF8MB4),
+				newExpression(CoercibilityImplicit, UNICODE, charset.CharsetLatin1, "latin1_swedish_ci"),
+			},
+			false,
+			&ExprCollation{CoercibilityImplicit, UNICODE, charset.CharsetUTF8MB4, charset.CollationUTF8MB4},
+		},
+		// binary takes precedence over latin1_swedish_ci at equal coercibility.
+		{
+			[]Expression{
+				newExpression(CoercibilityImplicit, UNICODE, charset.CharsetLatin1, "latin1_swedish_ci"),
+				newExpression(CoercibilityImplicit, UNICODE, charset.CharsetBin, charset.CollationBin),
+			},
+			false,
+			&ExprCollation{CoercibilityImplicit, UNICODE, charset.CharsetBin, charset.CollationBin},
+		},
 		// binary charset with non-binary charset.
 		{
 			[]Expression{
@@ -830,6 +909,23 @@ func TestCompareString(t *testing.T) {
 	require.NotEqual(t, 0, types.CompareString("\U000FFFFE", "\U000FFFFF", "utf8mb4_0900_ai_ci"))
 	require.Equal(t, 0, types.CompareString("æ", "ae", "utf8mb4_0900_ai_ci"))
 
+	// latin1 collations are byte-oriented, so the arguments are spelled as cp1252
+	// bytes rather than as Go literals, which the compiler would encode as UTF-8.
+	require.Equal(t, 0, types.CompareString("a", "A", "latin1_swedish_ci"))
+	require.Equal(t, 0, types.CompareString("\xE9", "E", "latin1_swedish_ci"))
+	require.Equal(t, 0, types.CompareString("a ", "a  ", "latin1_swedish_ci"))
+	require.Equal(t, 0, types.CompareString("\xDC", "Y", "latin1_swedish_ci"))
+	// Å, Ä and Ö sort after Z rather than folding onto A and O.
+	require.NotEqual(t, 0, types.CompareString("\xC5", "A", "latin1_swedish_ci"))
+	require.NotEqual(t, 0, types.CompareString("\xD6", "O", "latin1_swedish_ci"))
+	require.Equal(t, 1, types.CompareString("\xC5", "Z", "latin1_swedish_ci"))
+	// ß folds to nothing here, unlike in the utf8 collations above.
+	require.NotEqual(t, 0, types.CompareString("\xDF", "s", "latin1_swedish_ci"))
+	require.NotEqual(t, 0, types.CompareString("\xDF", "ss", "latin1_swedish_ci"))
+
+	require.NotEqual(t, 0, types.CompareString("a", "A", "latin1_bin"))
+	require.Equal(t, 0, types.CompareString("a ", "a  ", "latin1_bin"))
+
 	require.NotEqual(t, 0, types.CompareString("a", "A", "binary"))
 	require.NotEqual(t, 0, types.CompareString("À", "A", "binary"))
 	require.NotEqual(t, 0, types.CompareString("😜", "😃", "binary"))