collations: fix coercion semantics according to 8.0.31 changes (vites…

…sio#11487) (vitessio#11836)

Signed-off-by: Vicent Marti <vmg@strn.cat>

Signed-off-by: Vicent Marti <vmg@strn.cat>
Signed-off-by: Florent Poinsard <florent.poinsard@outlook.fr>
Co-authored-by: Florent Poinsard <florent.poinsard@outlook.fr>
Signed-off-by: Manan Gupta <manan@planetscale.com>

Signed-off-by: Vicent Marti <vmg@strn.cat>
Signed-off-by: Florent Poinsard <florent.poinsard@outlook.fr>
Signed-off-by: Manan Gupta <manan@planetscale.com>
Co-authored-by: Vicent Martí <42793+vmg@users.noreply.github.com>
Co-authored-by: Florent Poinsard <florent.poinsard@outlook.fr>

go/mysql/collations/coercion.go

@@ -208,9 +208,16 @@ func (env *Environment) MergeCollations(left, right TypedCollation, opt Coercion
 	if leftColl == nil || rightColl == nil {
 		return TypedCollation{}, nil, nil, fmt.Errorf("unsupported TypeCollationID: %v / %v", left.Collation, right.Collation)
 	}
+
 	leftCS := leftColl.Charset()
 	rightCS := rightColl.Charset()
 
+	if left.Coercibility == CoerceExplicit && right.Coercibility == CoerceExplicit {
+		if left.Collation != right.Collation {
+			goto cannotCoerce
+		}
+	}
+
 	if leftCS.Name() == rightCS.Name() {
 		switch {
 		case left.Coercibility < right.Coercibility:

go/mysql/collations/integration/coercion_test.go

@@ -135,8 +135,8 @@ func TestComparisonSemantics(t *testing.T) {
 	conn := mysqlconn(t)
 	defer conn.Close()
 
-	if strings.HasPrefix(conn.ServerVersion, "8.0.31") {
-		t.Skipf("Coercion semantics have changed in 8.0.31")
+	if v, err := conn.ServerVersionAtLeast(8, 0, 31); err != nil || !v {
+		t.Skipf("The behavior of Coercion Semantics is not correct before 8.0.31")
 	}
 
 	for _, coll := range collations.Local().AllCollations() {